Broadcast TV networks used to get first crack at all the best scripts in Hollywood. Nowadays, they are mere participants in a mad scramble.

TV-show creators this year are taking their pitches far beyond the networks, says Jennifer Salke, president of NBC Entertainment. "They are taking them to Netflix, NFLX -1.58% they are going to HBO, they are hitting every cable outlet," she says. "It just puts you in the position of being even more of an underdog."

Ms. Salke says NBC aggressively pursued one big comedy project from a major outside studio this season, but "the creators decided to negotiate with Netflix instead."

Growing demand for original TV series to satisfy broadcasters, cable TV channels and online video is creating a scarcity of writers, directors and even actors available to produce certain types of shows. Keach Hagey reports.

That one of the major broadcast networks would feel like an underdog highlights the increasingly competitive landscape in the world of television.

As the traditional broadcasters—Fox, ABC, NBC and CBS—kick off another fall TV season this week, an explosion of original programming from cable networks and online video is giving consumers ever-greater choices. The growing demand for new series also is creating a scarcity of writers, directors and even actors available to produce certain types of shows.

Many writers who networks would turn to for dramas, for example, are already tied up making shows for cable networks, industry executives say.

"The best writers are in demand now in multiple places," says Patrick Moran, the executive vice president of ABC Studios, part of Walt Disney Co.

"We're seeing everyone participating in this derby," says Kevin Reilly, chairman of entertainment for Fox Broadcasting Co. "It's at a fever pitch right now."

Some network executives say the broadcasters need to rethink their approach to producing programming around an official September to May season.

In Mr. Reilly's view, the broadcasters' current system is "insane": The major networks all race to buy scripts and produce 10 to 20 pilots each on the same timeline, hustling to get pilots made in the weeks before big annual presentations to advertisers in the spring, when they show off the ones they have picked up for airing.

That leads to artificially intense competition for talent in the last-minute frenzy. Mr. Reilly says there are times when actors he doesn't know—and has to look up on entertainment database IMDb—have six or seven offers broadcast networks and cable outfits.

"We're trying to lead the way to get out of that outdated system," Mr. Reilly says. He says the official TV season makes no sense when cable rivals are putting out their top shows at all times of year.

Network executives say they are answering the challenges by stepping up their original programming investment and ordering up more "limited series," closer in length to typical cable series. Shorter-run series make it easier to attract talent.

Viewership at the Fox network last season dropped 21%. In an interview, Mr. Reilly makes no excuses—calling the season "tough." Fox plans to spend $150 million more this fiscal year to develop new series—representing a 10% bump over the previous year in programming spending—as it adds scripted shows to the summer.

This week Fox will launch new shows including serialized drama "Sleepy Hollow," sitcom "Dads," and police station comedy "Brooklyn Nine-Nine." But then in the spring it will launch a comeback of the counterterrorism thriller "24,"' followed by mid-summer launch of "Wayward Pines" from creator M. Night Shyamalan.

(Fox network-owner 21st Century Fox FOX -3.04% was part of the same media company as Wall Street Journal-owner News Corp until June.)

Some network executives are weighing whether to skip pilots altogether for some projects, to attract high-quality talent. NBC, owned by Comcast Corp., CMCSA -1.76% ordered "The Michael J. Fox Show," a comedy debuting on Sept. 26 loosely based on its star's own struggles with Parkinson's disease, off the pitch— without so much as a script, let alone a pilot.

"We just jumped on board because we didn't' think we should be in a position to lose that project," Ms. Salke says.

Starz Chief Executive Chris Albrecht estimates there has been a 33% increase in the number of scripted dramas on television since 2010. Last season, he says, there were about 180 scripted dramas throughout the entire television ecosystem. "I've been in this business for decades, and it is more competitive than I've ever seen it," says Mr. Albrecht, a former HBO CEO.

With all the competition between new shows, it is also harder for winners to break through. "This day and age, 'The Wire' wouldn't make it beyond the first season," says Mr. Albrecht.

For the 2013-14 season, Needham Insights estimates that there are 70 cable networks that ordered their own scripted series. In 2002 there were "fewer than a dozen" doing that outside of the premium outlets like HBO and Showtime, estimates media industry analyst Jack Myers.

Time Warner Inc.'s TNT and TBS expect to boost original episodes by 40% this year. AMC AMCX -1.85% Networks on Monday announced it is developing a companion series for zombie thriller "The Walking Dead."

The proliferation of original shows elsewhere has taken a toll on repeats that broadcasters have long used to plug holes in their 36-week official season and throughout the summer. Over the past decade, ratings of prime-time repeats on the major networks among 18 to 49-year-olds have fallen 64%, according to Fox's analysis of Nielsen data.

"It doesn't make sense to dedicate airtime to repeat programming," says Benjamin Swinburne, a media and entertainment analyst at Morgan Stanley. "Everyone is increasing the number of programming hours and making it more of a yearlong schedule."

Write to Amol Sharma at amol.sharma@wsj.com and Keach Hagey at keach.hagey@wsj.com