I mean, sure, if you list all the positives about Amtrak and all the negatives about the others (without balancing out with the positives for the others and the negatives for Amtrak,) Amtrak would certainly look good.
However, Greyhound and the airlines both have positives, and Amtrak definitely has negatives. For the bus, it's cheap, usually more frequent and faster than Amtrak, and there's more stops (especially when including partner bus companies that you can buy tickets for online through Greyhound.) I wouldn't consider the seats especially narrow, though for whatever reason I can't get comfortable in them for a long journey. It's definitely doable for a few hours, though.
Speaking of which, they were able to bail us out when Amtrak had a huge snafu on my trip home last year. For whatever reason, Amtrak refused to hold our connection from the Capitol Limited to the Empire Builder, even though we were in the station at 2:13 PM and the Builder doesn't depart until 2:15 PM. A 10-minute delay, if that, would have had us on schedule and on our train home. Instead, Amtrak had such poor communication that even with signage and staff up directing people to the Builder, the train left without us! It didn't help that our SCA refused to let people off until he meticulously set off every carry-on bag onto the platform, even though we mentioned our tight connection to him. Amtrak offered us a room for the night and space on the next day's train, but that's still a 24-hour delay to get home. Luckily, Greyhound was able to cross-honor our tickets, and the Greyhound agent at Union Station was able to book us on the 3:45 PM bus from the bus stop to Minneapolis. We didn't get in until around 2:30 AM, but at least we were home that same day. The whole experience has kinda soured me from doing very-long-distance trips on Amtrak, though, especially if I care at all about my timetable to get where I want to be.
Also, I will say that Amtrak's equipment isn't as well-kept up as it should be, which can result in broken equipment, non-functional A/C, and some unpleasant experiences. Staff attitudes also range widely; I've had some amazing staff along my trips, and I've also had some terrible staff. Most are at least decent, and there's some that truly go above and beyond, but the percentage of less-than-decent staff is definitely higher than I've found on the airlines (or even with Megabus.)
As for the airlines, while the TSA certainly is annoying, it sounds like Amtrak Police aren't much better if you happen to fit whatever profile they're looking for. With Pre-check it's no worse than going through security at a sporting event or going into many government buildings. Meanwhile, the food I've had on board an airplane is generally edible (in fact, the snacks provided on shorter flights are generally name-brand snacks that I enjoy.) I've also had less delays on airlines than on Amtrak, and Amtrak seems to be just as prone to weather cancellations. My needs are also a lot less on a 2-4 hour flight than they are on a 24-48 hour (or more) train ride. A 2-4 hour flight, for me, only requires coach (and since I'm okay with any seat, I can use the basic economy tickets and save more money) and no food service, while a 24-48 hour train ride requires a roomette and meals, since I'll be sleeping on the train and going through multiple meal periods. Even with some rest time involved (which I don't seem to need much of, at least outside of what specific flight times I've chosen and the lack of sleep because of my choices there) it's significantly faster than the train, often cheaper, and has similar or better delay/cancellation recovery than Amtrak does. (Hey, if you want the Amtrak experience in the sky, choose the Big Front Seat on Spirit. Other than the TSA, it's pretty comparable to Amtrak coach!
)
That all said, if the airlines aren't your jam, then I'd definitely take Amtrak for a long trip over Greyhound unless the cost is prohibitive. It's definitely a more comfortable experience, and Amtrak seems to at least have the option of falling back on bus service if needed.
