Probably because many Japanese publishers (especially if they're smaller) can't handle PC development internally. Bigger publishers like Capcom and Bandai Namco do simultaneous releases on PC, they likely have established process for PC development already. (Capcom's Monster Hunter World is notable for not doing simultaneous release, as they admitted, the MH team had less experience with PC development.)
Do we know the stats on their PC sales percentage? I always assumed that Japanese games sold less on PC than a shooter or Western RPG would, lowering the enthusiasm to get a day and date solid port out.
I think big publishers do see PC as one of core platforms. I don't have concrete numbers right now, but you can see Capcom
boasting about PC sales of recent releases, or Bandai Namco and Koei Tecmo releasing almost everything simultaneously on PC. SEGA and Square Enix are still spotty, but progressing I think.
Also note that one of the big motivations behind PC release is Asia, especially China. For example, from
an interview about Atelier RPG series:
GamesBeat: Of the current consoles, what platform has the best sales — PlayStation 4 or Nintendo Switch?
Kikuchi: Well, for EU and NA, the number of sales on Switch is catching up with the PS4 version due to constant growth, but in Asia, the Steam versions dominates sales.