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.
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.
Not every developer designs their site for mobile. Some design it for desktop, others for apps, some for phones.