From what I’ve heard from former Google employees, it’s primarily because Google has a weird incentive structure for its engineers which puts all of the weight on launching new products, but not improving, maintaining, or sustaining those products. For most Google engineers, the only way to advance one’s career is to be part of the launch (or major overhaul) of some product (e.g. Google Glass, Stadia, Google+, etc.).
The more products an engineer is part of launching, the easier it is to get promoted, get pay increases, and advance one’s career. There’s no benefit to staying on, so the ambitious engineers will only stick around until the product launch counts toward their promotion cycle, then abandon the project shortly after to join up with another product that’s still in development. The former Google engineers called this the LPA cycle - Launch, Promo, Abandon. This is also why so many Google products stop receiving meaningful post-launch updates. Most of the engineering team has abandoned the product. It really doesn’t have much to do with Stadia in specific.
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