I don't wanna cause a misunderstanding here, though - the story of FF9 is that for a time (like with 13 later on) they weren't sure if they were going to do it on current or next-gen machines. (Early PS2-based FF13 screens tests are out there, testing the HUD, combat etc using FF10-2 assets, for instance!) In 9's case they decided relatively early on to go for PS1. So a very small number of assets existed in a high quality, not all. It's not like this could be a flicked switch.
Also - older SE games (pre, say, FF12) have pretty bad code rot where builds weren't stored properly and things - that's why KH1.5 took a while, for instance, as all the old assets weren't stored at their base quality and as such, lots had to be rebuilt.
Long story short - I'd love to see FF9 rebuilt with a slightly faster battle speed, higher resolution assets but no other changes - but frankly it's not an easy task at all.