1. AWS having been around longer, and having been vastly bigger and more widely used than GCP for the entire lifespan of both products. AWS is just more expansive/evolved from having piled on more features for more customers over more time.
2. AWS virtually never makes backward-incompatible changes. Every feature of every service and every version of every API since its inception is still functional and presumably will remain that way forever (with a very minimal list of exceptions). So while it's certainly easy to imagine how the entire thing could be streamlined from a blank slate, it's just not how AWS operates.
Meanwhile, Google is absolutely notorious for wantonly EOLing products; see: https://killedbygoogle.com/. Besides the extensive list consumer products (RIP Reader), there are quite a few examples of APIs and cloud services that have been axed. I'm not very familiar with how/if GCP specifically has been impacted by this trigger-happy EOLing, but frankly I think you'd have to have a huge appetite for risk to trust that GCP is not going to pull the rug out from under your business on a whim.
The upside of the never-make-backwards-incompatible-changes approach is that as a customer, I don't have to worry about proactively monitoring for breaking changes to my applications/infrastructure and pre-emptively emergently refactoring/re-architecting them to avoid downtime. I have various little AWS CLI scripts that have been running untouched for years, and they still work. That's a tremendous timesaver and cost-saver whether you're just doing side projects for fun or running enterprise IT.
The downside to this approach is that the product as a whole is an ever-growing spiderweb-snowball of complexity, by design. While some new services/features get added as simplified alternatives to existing offerings (i.e. Amplify, Lightsail), they still exist in an ocean of cruft, and it takes some level of expertise just to be aware of their existence and tease out exactly what aspects they simplify and how they interface with the rest of the ecosystem. If and how AWS resolves this problem dynamic over time remains to be seen; it certainly is a pretty big Catch-22 from a high-level perspective.
So, in short, while GCP is inarguably a more streamlined and user-friendly platform today, it is a bit naïve to make a blanket comparison to AWS on this basis. There are good reasons why AWS is the way it is, and I wouldn't necessarily bet on GCP maintaining its comparative sleekness over time as it may eventually fall prey to the same problems. (Nor would I bet against them avoiding/solving these problems either - there are a lot of very smart people at Google.)