>>106904756 (OP)
Because on the web the user is in control of everything that runs on the client, and can modify and extend functionality with plugins to block advertisements, preserve privacy, etc.
Section 1201 of the Digital Millenium Copyright Act makes it a felony to bypass a digital lock (which you have to do to modify a mobile application), punishable by either a 5 year prison sentence or a $500,000 fine for a first offence.
An app is just a website that's been wrapped in the right kind of intellectual property law to make it a crime for you to defend yourself while using it.
This is why every major tech platform wants to shunt you into their app when accessed from a browser on mobile, in some cases even forbidding use from a mobile browser.