Because if you take some of the letters of "Gabe Newell" you spell the word gelable.

Which is colloid in which the disperse phase has combined with the dispersion medium to produce a semisolid material, such as a jelly.

The boomer from Left 4 Dead 2 is gelable being.

And since Left 4 Dead 2 was 9 years in development (which was worth the wait) Gabe Newell took the time to put his heart and soul into the boomer.

Later, Gabe wanted to become closer to his characters so he started eating people (Which slowed production that's why tf2 updates and portal 2 was pushed back)

And looked somewhat like the boomer

Unfortunately he overdid it and if he replicated the boomer in his own form it would take up too many polygons in the model.

So Gaben is forever made fun of. Gabe Newell gets depressed so he eats someone from the episode 3 production every time someone makes a fat joke about him. This literally proves that "Every time someone makes fun of Gabe Newell, he delays Episode 3 for another week" is true.

And that my friend is why Gabe Newell is fat.