This is a consequence of bad internal economics and incentives. Google PMs and engineers are incentivized to create new products and ship them by quarterly deadlines (perf/promo cycles) rather than align their teams towards a cohesive user experience. Thinking this way would occasionally cause teams to, gasp, not build a product they wanted to build. Everyone needs to ship something, ideally something new. After all, you don't get promoted for deciding not to build something. I've seen awful products/implementations go out the door and get killed or reimplemented soon thereafter. And people still list these as "achievements" on promotion packets (and they get promoted despite the product failing!).
The company is kind of trying to address this by changing promotion criteria, but it's culturally ingrained and won't change for some time.