In general however it is a compromise. At least "true" sleeper cars often have a (shared, small) shower, but yes, no comparison to equally expensive hotel rooms.
That's a super-expensive luxury cruise train that only does irregular (and mostly circular) tours - it's the equivalent of the Venice Simplon Orient Express. There's one scheduled sleeper train left in Japan, the Sunrise Izumo/Sunrise Seto.
Unfortunately even though we tried to book the train as the first thing after arrival, right on the Narita train station ticket office, it was already fully booked. :P
At least we managed to book the marvelous SL Hitoyoshi steam train from Hitoyoshi to Kumamoto over Hisatsu line. We were actually double lucky on that one, as SL Hitoyoshi is actually out of service right now as floods in July 2020 washed away key bridges on the Kuma river, cutting the rail connection between Hitoyoshi and Kumamoto, until the bridges are repaired...
Still we did go to the very inaka Izuma anyway and spend a very nice old school and almost mythical four days there.
And what we did about that missing accommodation? We booked a night in a ryokan in the splendid Kinosaki onsen town instead and it was indescribably good decision! :)
It took as like 6 hours by multiple small local diesel trains to get from Izumo to Kinosaki, possibly the only tourists to arrive from that direction in quite a while! :)
And on the very elegant Kinosaki station we even discovered a special gate reserved only for guests arriving by one of those luxury cruse trains - namely Twilight Express Mizukaze: https://en.m.wikipedia.org/wiki/Twilight_Express_Mizukaze
Likely inspired by the special gate only ueed by the Emperor on Tokyo station. :)
I would not want to have strangers sleeping next to me.
Bit have you tried the "Book ticket" button?
For me, it simply goes to the start page.
So I still don't know how to find the routes that offer these options.
That is, "Most Nightjet trains use Comfortline sleeping-cars built by Siemens in 2003-2005 for German Railways' City Night Line sleeper trains." and "Below: Comfortline sleeper layout. All compartments can be sold as a single, double or triple."
Every time I try to book a trip, the single compartment is grey and says that this option is not available for this trip.
I picked 14 April 2021, leaving Wien Hvf (U) at 20:13 arriving Berlin at 08:55. "Sparschiene Nachtverkehr Nightjet + Anschlussticket Nightjet." "Sleeper Bed", "Compartment with 1 bed (Single)".
€ 216,00.
Bear in mind the comment at https://www.nightjet.com/en/ : "Due to the travel restrictions caused by the corona pandemic, we are reducing the Nightjet traffic to probably 8 February 2021 ... As a precautionary measure, no bookings are currently possible for the affected trains from January 10th to March 24th, 2021", confirming johannes1234321's comment "During Cornona this probably is different".
And I couldn't really spontaneously take a train to the center station where I would then catch a night train because of course I have to make sure I get there in time for the train going to where I want to go.
I love trains. They often go every hour. You can spontaneously take them. Right from the center. And you arrive right in the center. And you can freely chose your seat. So you can pick a pleasant neighbor. Or just change seat if your neighbor annoys you.