There are not yet enough PS5 in the open to even have enough install base to get on this list yet. There are 172k PS5 (I'm leaving out the discless version since those can't contribute to the software numbers) and you need ~65k sales for a game to appear on the top 50, which would ba an attach rate of over 30%. Even for Nintendo titles, 30% are a lot and very hard to reach, so it's even more difficult for any other brand of games.
I'd look again in 3 weeks, at the end of the year. By then, the install base should have increased to an amount where a PS5 title technically could show up - but it would still be very difficult considering how slow the PS5 software sales seem to be so far.
I cannot let this explanation stand as it is. Attach rate drops with higher install base. The reason for this is simply, that a higher install base comes from different people buying into the system. You only have platformer fans? You get a WiiU. But that also leads to pretty high attach rates, as most of the install base share the same preferences and if you get a good game for that, pretty much every one owning the system buys that.
Every system goes through a phase like that. And that is launch. At launch you don't have a broad diverse spectrum of games, you have only so much. That leads to Breath of the Wild having nearly a 100% attach rate at the beginning. That dropped as the Switch base diversified.
PS5 not having this (in Japan) probably means that no title is a system seller there. Which is in line with the bad sales.