Very easily, and because so few people know it is possible, it's a rather nasty vector for cross-site scripting attacks.
Also you seem to have no idea about where the web is headed or have heard about responsive design and SPA.
I'm well aware of responsive design. I think it's an abomination, because all it does is make it take two page loads to view your site instead of one, by ensuring that I have to first load your broken mobile site, then click the "full version" link. Every single freaking time I end up on a "responsive" mobile version of a website, I find myself locked out of features that I regularly use, and end up having to switch to the full desktop version of the site.
And SPA is even worse. If your site loads significantly faster as a web app, there's something wrong with your site. 99% of the time, most of the resources should be shared across pages, and only the text of the page should be changing. There's usually not an appreciable difference between the "load the full page" case and the "load the body of the page" case from a performance perspective unless something is very, very wrong. There are exceptions, such as storefronts that use precisely the same page layout for every page, but these are exceptions, not the rule, and even then, the extra savings in initial page load time just result in a customer sitting there wondering why there's no data on the page, and thinking your site is broken. The real problem is that every web engineer thinks their site is the exception to this rule, but most of those engineers are wrong.
More to the point, if I'm accessing your site often enough to care about performance, I'm going to download your native app instead of using your mobile site, because it will always be much, much more functional, with fewer limitations, more features, and better performance. If I'm going to your website, it's either because I don't care about performance or, more commonly, it is because your native app is missing features that are only on the full version of your site. Giving me a mobile version won't help with the second case, and the first case is largely unimportant for everybody but the site designers who are trying desperately to shave off a few bytes from their data bill.