Because then you don't have to pass a huge framework library down the wire along with all of your custom code? Look at it the other way, if you don't get a big advantage over web components by using React, Svelte, etc then why would you use them? Web components are natively supported by modern browsers. Using a fairly simple base class like Lit-Element to provide some template niceties and lifecycle hooks gives us all the extras we need. We've figured out a really nice top-down functional state pattern for data that eliminates the need for two-way binding and requires very little "framework" level code - it's probably like 100 lines if you take out whitespace and comments.