Sound / speaker physics;
For deep, low frequency sound - you need to move a large amount of air
For medium frequencies - a speaker that can move a moderate amount of air but remain agile and fast
For high frequncies - you dont need to move as much air, just that the driver is able to move very quickly
Thats as basic as it gets. There really is no wizardry in audio, all you are doing it moving air. So with that in mind that is why you would traditionally have a large driver, a kid range driver and a small driver.
A soundbar is typically long, flat and not very deep; so you wont be getting larger drivers in it = loss of bottom end
Thats why 'subwoofers' are usually used in conjuntion with them but what those soundbars use arent what you might ordinarily call a subwoofer as they usually lack the size, throw and power of something actually capable of going into sub territory; they are usually more in between a mid range and a sub and fill in the lower freauencies the sound bar cant reach.
I have always maintained a good 2.1 setup is better than a shit 5.1 or more setup. A lot of soundbars are good with spacing and direction and can give the impressiklon of surround sound but for me; punchier, more dynamic 2.1 is a better solution that thinner, less dynamic sound from multiple directions.
But its entirely about what you can fit in the room, what works with your entertainment cabinet etc.
You just have to make the decision on what you're prepared to lose or what can be realistically achieved in your living area
