There are sets that have modular designs, but not many - a few of the Loewe models, I think still have them, for instance.
A lot of this seems much worse, of course, given that there have been quite a few changes recently, after a relatively stable decade or so in digital TV; in the last couple of years we've seen DVB-T2, and MHEG-IC, plus the introduction of H.264 and multi-channel surround in tandem with HD services, and growing interactivity.
But even so, remember that making tuners modular and easy for users to replace is going to introduce more connectors, and potentially increase cost while decreasing reliability, even if only slightly in each case. And for what? So that a few people will be able to swap out a tuner module for a slightly better one.
Honestly, while the readers here may agonise about that, it's going to be a pretty small minority of people. Sad fact is that, when it comes to AV gear, "good enough" is all that most people want. That's why they put up with VHS, why hardly anyone buys hifi separates, why Sony's Profeel didn't take over the world, etc etc.
Many people won't be spending £2500 on a TV. They'll spend a lot less than that, and many of them will happily buy last year's model if it's going for a song, just so they can watch Eastenders in HD. A lot of them do have tuners that are separate too - in a Sky or Virgin box, for instance.
In some cases too, it may not just be the tuner - swapping tuners might get you an upgrade from DVB-T to T2, but it's not necessarily going to conjure up an audio processor with Dolby transcoding, an H.264 decoder, or a processor that can handle YouView's graphics, or a DRM chip for some internet TV services. To avoid replacing the set, you'd need everything modular, not just the tuner.
Most people are just going to settle for "good enough." Again.