You shouldn't be asking for a couple of trains between 8pm and midnight. They should be every half hour, or more! Midnight until 6am on Saturday and Sunday could be every hour or two.
(I live in Copenhagen, half the population of Seattle. Commuter trains run every 20 minutes from 5-24h, every 10 6-18, all night on Friday and Saturday nights. Other trains also run, and the metro is 24/7.)
There is so much thing to undo and people are so overly sensitive of the value of their property it is nearly impossible or would take centuries to fix.
Honestly I think busses are far superior due to not having so many dependencies.
I've ridden the train in Copenhagen. The stops are near things! Stores and parks! Apartments and hotels! Shopping malls! Like, actually right there! There's a few stops like that in Seattle, but they're also the ones with junkies nodding off, con men "looking for some gas money", and other anti-social behavior.
But you are absolutely right, Tacoma and Everett are 3rd and 4th largest cities in the state and are in the same metro area as Seattle. The north line is actually worse, only 2 trains a day (plus 2 Amtrak Cascade trains) even though there is major Ferry terminal connection at Edmonds (and a smaller but significant at Mukilteo). The Sounder service is dismal and needs to be improved (thankfully we are getting more service on the Amtrak Cascades which runs the same corridor but goes all the way to Portland and Vancouver BC. but not nearly enough)
That said, the bulk of the population in the Seattle metro area is serviced with the Link light rail. They are almost done with an extension east to Bellevue (turns out building rail on a floating bridge is hard) and eventually it will reach both Tacoma and Everett. Then they will get the frequency they deserve.
As for me, how do I get home. My ferries actually run all night (although with up to 3 hour gaps in the middle of the night) and frequently enough. If there isn’t a bus on the island, I usually just call my partner and she picks me up at the terminal, sometimes I also meet someone I know in the ferry.
The funny bit is that I actually immigrated here from Europe, and my tiny European island in the North Atlantic actually has way worse public transit system as Seattle. So for me this is a huge improvement. I’m actually going home to visit and am planning to see a friend of my play at a concert it a town 40 min from the capital, but there are no buses, so I actually have to call my mom to pick me up when the concerts are done.
But the population of the whole country is 1/100 of Seattle. Buses in some place during the day, and taxis at other times, is proportionate to what's available in Copenhagen.
I grew up in a small village in England. Choices for a night out were staying over at a friend's house (much preferable) or splitting the £15 taxi between several teenagers, which was a decent price as far as our parents were concerned. (Or walking for two hours and keeping the money, don't tell mum.)
Not quite that tiny. Iceland. The public transit system inside the capital area is decent (probably better then Spokane’s despite being similar of size) but after midnight it is really nothing, and on the weekends the frequency gets kind of bad (not North America level bad though). However ones you go outside of the capital area the system really sucks. Despite receiving over a million tourists which more then justifies an airport train, there is only a bus every couple of hours to the airport and adjacent towns (with a pathetic shelter).
Most US towns are basically uber/taxis only at that time. I live in Austin where you could theoretically be one of the lucky 1% of the population that both lives near a train station and wants to go somewhere near where that train station is. Even then you can’t use the train past something like 11pm on weekends. Capmetro’s hours are a pretty decent source of consternation for us here
The Seattle area has a newer, dedicated light rail system (Link) that runs every 5-10 minutes from about 5AM to about 1:00AM. The stations are located much more conveniently and frequently, because they were placed based on where a commuter train should stop, not trying to piggyback on existing freight lines. There still isn't as much parking as there should be, but at least the stations are placed so that more than a tiny handful of people can get there without driving and parking.
The South direction is a little better (and is improving even further with more Cascade runs on the horizon) but it is the same story. For example, Pyuallup and Sumner are both decent sized towns wit centrally located station, which won’t get light rail and the 1 line won’t reach Tacoma until 2035 at least. There is also a decent transit center at Lakewood, however I would argue it should probably be better to run the 620 all the way to Tacoma, from where Olympia people could jump onto a Sounder or light rail.
If the Sounder would run like the ferries do (say every hour until like 8PM and then a couple more trips before midnight) it would really take the hassle of going to the city for much more people than robo-taxis ever could.