The FTA tv tuner in the t-box is "mediocre at best", certainly not the "best" like I stated. And last I heard the rebadged Roku hardware struggles to provide a consistently fluid experience in their software. The region based locks for apps in their target audience area is crazy too.And yet here I am with my Roku powered Telstra TV box.
Telstra is actually a pretty good example to illustrate my point. Many of Apples customers live in countries that are still using analog broadcast. And there are ~10 different broadcast standards the tuner would need to decode to work worldwide. The reason it illustrates my point is because I would venture a guess that 95%+ Americans (United States) haven't even heard of Telstra in any of the businesses variants.
If Apple were to add a tv tuner they would need to add a ton of SKU's (which can drastically increase cost) and/or make a box with everything that would end up being huge in physical size and larger in cost.
Also consider the tech support cost and software development cost. Apple would need to rely on your aerial quality and position PLUS reception in general to provide a good experience.
Overall this isn't much different than installing FM radio receivers in iOS devices like other smartphones have/had. Without an external aerial the quality of performance is limited making alternatives (actual FM radios) much better options.
EDIT: BTW I'm not knocking the t-box nor am I glamorizing the ATV as we both know it has its shortcomings. However if you have good experience with it then by all means use it. However even in Telstras most functional regions with great FTA reception for your favorite broadcast the t-box won't provide you with the highest quality service that is available with specific built equipment.