Yeah - on mobile you pinch your fingers together and spread out as a gesture to zoom out. You can go up and down by flicking your finger on the screen. It's still the same. The input methods are different but they have equivalents that are more or less standardized by now.
I feel like mobile design has slowly become "do it the designer's way or you're wrong" and give it another 5-10 years before it swings back around to "Oh god, what were we doing taking away the user's choice for all those years?"
Not really. On desktop there are different ways of controlling the zoom level, it re-flows if you adjust the font size or if you change the desktop resolution. Sure you can root around in the menus on your mobile phone to make the font size bigger but nobody does that. We just zoom without the re-flow.
Actually css-tricks did investigate how many people adjust font size on mobile and it wasn't quite 'nobody' but it was rounding error. When it comes to desktop an anecdotal walk around the office will give you the answer - lots of people change their desktop resolution to make it legible.
With a full screen mobile modal you just want to get an interaction from the user of the yes/no variety, if keyboard needed then that will be for a simple code or name that is possibly auto-filled. There is no more to it than that - it is a modal.
On the desktop the 'correct' tool for this job is actually the 'alert box' but these got abused and 'designers' complained that they could not 'style' the things. Hence we now have frontend developers load loads of useless libraries and spend however long styling these things so that people can subscribe to newsletters they will never read.
Hence, for the use case of 'modal dialog boxes' the rules of regular content do not apply. You aren't going to get extra doodads that distract the user away from the task in hand. It is just not needed.
What next, vending machines where, prior to buying your snack item you can choose fonts and colours of the 'insert coin' display?
On the smaller screens - I take the lack of reflowing as good design. I wouldn't want to the site to reflow when all I was trying to do was bump up the font size to read something easier as it would make it impossible to tell where on the site I am. Horizontally panning on a phone isn't nearly as annoying as it is on desktop - and the difficulty of horizontal scrolling is why page reflow is a good thing for desktop: it prevents the need to scroll horizontally.
I'd never adjust my phone's font size - I pinch zoom, literally, every page I visit that allows me to do so on my phone because it's a very selective zoom. It allows me to focus on one part of the site, read small text in just one section (where the text in other sections might be fine), or makes it easier to click a link, button, or what have you. I'm not talking about adjusting font size - I'm talking about zooming. Almost nobody mucks around with settings, it's something like 5%~ of people change the default settings and the other 95%~ never change a thing.
Being able to change the font size of a modal is about usability [0]. If I can't zoom in on the tiny font because they've broken pinch zooming in order to place the modal on the page I'm closing the site.
ps. The actual proper way to do this is with a dialog modal [1]
I feel like mobile design has slowly become "do it the designer's way or you're wrong" and give it another 5-10 years before it swings back around to "Oh god, what were we doing taking away the user's choice for all those years?"