Because the PS5 isn't going to get as many exclusive Japanese titles as the PS4. Most Japanese games are either going to be multiplat or Switch exclusive.
I don't see why that would be the case, at least not to the extent that it will significantly affect sales; Sony like to moneyhat and plenty of Japanese devs have shown no reluctance to go all in on Playstation, even with the Switch dominating the market.
You're thinking assumes that devs can make a game in a really short amount of time. In reality, most Japanese games take about 3-4 years time to develop. The third party games that are being released this year are based on how people thought the Switch would sell in 2017-2018 (i.e. decently well, but not super amazing). Based on that alone Switch is getting a multiplatform version of most Japanese titles. We haven't seen yet seen how devs have responded to Switch's 2020 sales. PS5 in 2023-2024 will be lucky to get any third party exclusives that target Japan.
Game makers like to make a profit on their games. Switch is selling like the DS, but it's capable of playing big budget titles like a home console. At this point every Japanese game developer is going to want to put their games on the Switch. It has everything they want. At the same time, Sony has mostly given up on Japan. They aren't going to moneyhat a game from a series that has mostly just sold to Japan in the past.