Because the performance of existing web frameworks is... _absymal_. They are nowhere close to how fast they can be. And I don't mean slower by a bit, I mean close to a factor of 10x. My in-house framework can serve 600-700k requests/s on a single core, while beast (the best c++ framework i could identify) does 250k at best. nginx/apache and other prebaked servers are even slower, at 100k req/s. Tells you all you need to know.
If I were to open source a project these days, I will definitely have my progress be a clean, huge initial single commit. Too many busybodies trying to piece together code forensics to say something is vibe coded or not.
If new free projects had to have a "100% artisanal" pledge I wouldn't even consider starting one. Hand-crafting the core design is still superior. Not delegating the grunt work for copy-paste level stuff is just wasting time, though. I don't require the circuit boards in my electronics to be hand-drilled either.
Performance is probably the only angle that has some potential but only short requests, the db and other external IO/services is a bigger bottleneck.