I read an article on Soundbars that said that TV speakers have gotten so bad because the TVs have gotten so good.
This.
Lower frequencies, the ones that give sound "fullness" or "depth" (in quotes because, subjective) need X amount of air volume per driver to work effectively. As TV form factors grow ever thinner, there simply isn't enough air volume in the case to accommodate sufficient speakers for any more than vestigial sound. The result is tinny, thin audio that's generally little better than transistor radios from 50 years ago. As has been mentioned, some TV mfgs are addressing the problem by putting the speakers/electronics in a remote-mount case. Others just presume you will have some type of sound bar or H/T system, or that you don't care about sound quality at all.
It also doesn't help that many channels are transmitting in some type of 5.1 digital sound format. The dynamic range of digital sources is well beyond what most internal speaker systems can accurately reproduce. Add to that the downmixing for 2.0 stereo (the TV's two internal speakers) not being handled very well. It all contributes to crummy sound quality.
A sound bar, even an inexpensive one, is light years better than most modern TVs' built-in audio (get familiar with your sound bar's delay function, it usually take some twiddling to get the sound synced). Some wireless headphone setups these days are also darned impressive. I'm an old-school movie hound and have a full H/T system, but that's me. It's waaaaay overkill for anyone not into movie soundtracks like I am. If I have house guests I don't want to disturb I'll plug a long cord into the headphone jack and use earbuds. While they don't supply thunderous punch-you-in-the-chest LFE like my two 15" subs can provide, it's still very, very, very good sound.
Brad