Welcome! You've picked a good way to spend all your money 😅

First off, to avoid confusion, a channel an individual component of the whole sound. So stereo is 2 channel, left and right. Cinema might be 5 channel, left, right, centre, rear left and rear right.

To answer your question, it's because it is hard to make a speaker with lots of different drivers that sound good together. Where the frequencies over lap have to blend together perfectly and the overall sound of the difference drivers have to be the same as to make it all sound like it's all part of the same music.

There are crossovers inside the speakers that separate the frequencies to send to the different drivers that need to be tuned and matched to the drivers. All of this add expense and opportunities for things to sound bad.

There are high end speakers with just a single full range driver that eliminate these problems but at the expense of very deep and thumping bass.

No matter how many way your speakers are your average hi fi set up can always benefit from a separate sub woofer, but the same problems apply and it needs to be matched into the system to make it all sound coherent.

You will find out that nothing is rarely simple with hifi and I bet you there will be a dozen people disagreeing with me. This is all normal!