Driverless-car ventures are set to attract some eye-catching valuations over the coming months, but this isn’t a sign of technological maturity. The road to autonomous driving is actually getting longer.

Uber is looking for external investors, including SoftBank and an unnamed car maker, to take a stake in its self-driving vehicle unit, The Wall Street Journal reported last week, citing a valuation of between $5 and $10 billion. More surprisingly, so is Alphabet’s Waymo, according to a report in The Information, a Silicon...