It can end up being just as expensive as flying, let alone driving, enragingly. That said, if you’re willing to go off hours, it can way cheaper than either. If you’re not just going city to city though the pain of traveling to and from the station might remove the ease of use aspect.

Long story short, there are a lot of reasons. Plus, the trains are almost always full.

Edit: and I say this as someone who’s ridden Amtrak to get back to my parents exclusively for years, and who lives literally two blocks from my local Amtrak station.