Making the font size adjustable is not easy to do reliably. Once the screen layout is settled for a given size and the data it presents, making the font larger means that the layout can easily be mucked up by longer data (just an example). I believe it would both substantially raise the amount of testing Tesla would have to do AND increase the number of bugs we all see. I base this on many years of software and UI experience.
When I first got bifocals (now over 70, so quite some time ago), I set the distance of focus of the reading part to the full distance of my outstretched arm+hand. I did that so I could easily read everything on the instrument panel of my plane. As a result, I find the Tesla screen text to be perfectly readable in all circumstances. The only down side is that, if working on something extraordinarily small (like electronics or tiny text) I need a bit of extra lens. I've found several ways of adding a diopter or two that suit me and consider the trade-off a good one given that I can see better 99+% of the time with just bifocals.