Well, it's because PSX is simply designed as such, extract and play.
In reality though, all PS1 emulators do have the capability to run with 60FPS. However, it differs much with the plugins and settings as well.
I have tried all of the PS1 emus (Even Bleem and VGS) and all of them can run full speed given a powerful PC. My hunch is that it is only caused by incorrect plugin settings or a slow PC.
..... Short answer, you just need to try another plugin and/or reconfigure it correctly.
