With the way Xbox is cratering and even Switch dropping more heavily now, PS5 should be comfortably ahead of PS4 by a good bit more than 8%. So my only answer to why that isn't happening is because a bigger portion of those would-be PS5 buyers coming from Xbox and Nintendo are going to PC (and to a lesser extent, mobile) rather than PlayStation.
So it's not like Sony aren't losing potential sales: they are. It's just not enough to have PS5 tracking behind PS4 in the US market. Since you brought up Japan though...well there for the year, yes PS5 is tracking behind PS4 just looking at CY 2024 vs CY 2017. The drop in PS5 sales in Japan from last CY to this CY has been absolutely significant, and it's still not out of the question they could be looking at 10K-15K weeklies as the norm going forward.
Is it the PC ports causing this to happen? No, not
exclusively. And again I would even say that for now they aren't the biggest factor: pricing and economic factors outside of Sony's control are likely the bigger culprit as to why, for example, PS5 is "only" up 8% PS4 in the US (I'm not trying to say this to downplay it being up, just saying it could and probably should likely be even
more up vs PS4 considering the collapse with Xbox and the slowdown with Switch).
However, and like I've been saying for a while, the PC ports certainly aren't playing a non-zero factor in this. We'll see to what degree they're having an impact on longer-term adoption rates once the PS5 Pro releases, provided Sony/SIE continue their porting strategy the same as they have up to this point (or get even more aggressive with it). You're eventually going to be wrong in underestimating the long-term impact of their PC porting strategy on PS console sales (and even B2P, subscription & MTX revenue), Mibu.
Or at least, I have a very strong feeling you will be. But again, that's only if things continue as-is or the pacing & volume of ports picks up even more going forward. Heck, we might get a glimpse into if that happens later tonight
.