Because modern web is bloatware. Too much javascript, CSS, ads and cookie popups. A phone’s hardware and internet speeds are generally not as fast as a desktop. So, it takes much longer to render on a phone.

Also, a lot websites nowadays deliberately make their mobile web experience shitty (cough ** reddit cough) to force their users to install their app.

At least some of it is shitty companies either neglecting to optimize or even outright sabotaging their own sites to try and force you to install their data harvesting app so they can sell more of your data for profit.

The rest is just lazy devs not optimizing because they’re lazy and/or underpaid and overworked.