Your garage project also doesn't need to support 100's of random features some product manager thought are critical but nobody uses. Quality issues are exponential to the complexity of the software. Out of those 1000's of engineers there will also lot of variability in terms of productivity and quality. Some engineers are likely doing most of the heavy lifting. Some spend all their time in meetings. Some engineers might be working on their novel or startup while sitting in the office. Extremely unlikely they're all operating at the same level. With 1000's of engineers there's a lot of effort synchronizing those engineers, communication overhead, people stepping over each other's work etc. The mythical tower of babel: https://en.wikipedia.org/wiki/Tower_of_Babel

Lots of reliable/production software was built by teams much smaller than 1000's. Things like operating systems (e.g. the Linux kernel), compilers, tools or libraries, come to mind. Some even by single people.

That's not to say that all weekend garage projects are better than what 1000+ developer teams produce but don't knock the ability of small teams to do amazing things. Twitter isn't exactly the pinnacle of engineering accomplishment either. I'm pretty sure Twitter suffered from bloat similar to many other large tech companies. Elon taking a sledgehammer to that is probably not the best approach though. That said some people were saying the whole thing will fall apart in days, and it didn't.

[EDITed for typo]