I've got a big holiday planned in the US for the summer across a number of cities but I'm a total noob to flying domestic in the US..

In my homebase and elsewhere I tend to prefer legacy carriers- the perception that they're more reliable, all-in fares etc.. I looked up Delta/American/United domestic across my city pairs and I notice that I can't pre-pay for check baggage on domestic coach.

Have the US legacy carriers adopted a low-cost model or am I missing something here?

Are they also more reliable?

Thanks in advance