They can, they dont want to. Both MS and Sony know people will buy them nonstop.
Sony had psp. What stops them from making that? The writing is in the wall. They are chasing euro. dollar and pound. They are giving up on japan, simply because they cant compete with nintendo hardware.
"buy them nonstop"? Specially Xbox consoles? lmao. Have you been checking Xbox hardware sales?
In countries like France, Xbox Series is the worst selling Xbox console launch aligned right now. And PS5 literally sold 4M less last year than Sony initially expected. You didn't see them decreasing the price did you? They actually increased it.
About releasing a portable device out of nowkere...it makes no business sense, specially if it's to try and get sales from a single country. They also probably don't want their studios spread between two completely different pieces of hardware dude.
Do you really think Sony's suits (or anyone with a decent vision) is going on a meeting thinking like "hey we should make a portable so that we sell a few more units on Japan" when they have the entire world to sell PS5?
I'd rather them do what they want to do, if Japan doesn't care...that's on them. They are more focused on finding a market in places like Korea, they are starting to invest in india, etc
What even is "giving up" on Japan anyway? Cause clearly they release games that cater to that market. Also, Japan is as far as i know the 3rd biggest market for PS consoles after USA and UK. It's not like Sony would give up on such a huge market for them.