- the core if the web has been, is, and will forever remain a system of displaying static text with a couple of images. You as much as look at it funny, and it needs to re-layout and re-render the entire screen
- there's no standard library to speak of
- there are no standard widgets and controls to speak of (https://open-ui.org/ is twenty years too late)
- there are no easily accessible ways of handling state
- almost every single API that the web provides is either underngineered, or overengineered, or badly designed, or all of them together
- the web rarely provides a comprehensive and/or a generic solution or an API. It's a hodgepodge of one-off hacks often needed to cover deficiences in other hodgepodge hacks
So, to make the web even remotely app-like, you need to solve for all these issues, and that's before you even start writing a single line of app logic.
And on top of that you have language evolution and people wanting to use features of tomorrow today.