You underestimate how many people buy that ~3k TV on credit and for whom an extra ~150 bucks for an Apple TV is actually non-trivial money.
In fact that's the reason these ads are there - a lot of people can't be convinced to use an external device to escape them. If everyone used Apple TVs/etc, there would be no built-in ads because it's just not worth the trouble.
If I were in that position I think I'd buy a TV that's lower priced but still quite good ($1.5-$2.5k) and spend the difference on a good streaming box. The value per dollar is much better that way.
In fact that's the reason these ads are there - a lot of people can't be convinced to use an external device to escape them. If everyone used Apple TVs/etc, there would be no built-in ads because it's just not worth the trouble.