Perhaps it's hard to explain. Original PSX games were written with PSX and its hardware in mind - the whole platform, PSX CPU, GPU, audio, separate geometry transformation engine, data decompression engine and a lot of other things that are not present in modern devices. In short they are just totally not compatible with each other.
So to run PSX games on a modern PC/Mobile phone someone would have to write a program that would interpret every single element from the original PSX game and write complex algorithms to transform it to something that a modern CPU would understand, decode and attempt run by itself - at that point it tries to process tasks that on PSX were done by multiple separate pieces of hardware that were done exclusively to serve their purpose and do part of the job. So a modern CPU struggles to break down complex technologies to simple functions that it's able to run by itself, and then attempts to run them. That's a lot of work, even considering that an emulation software was perfectly written and doesn't add even more pointless work to be done.
To make it less confusing a pretty close real life example would be like having 20 Spanish-speaking experts (each of different trade) and a chief (PSX application) giving each of them a task that is exactly fitted because he knows exactly what they can do and how to communicate with them to make the task perfectly clear. Obviously the task is given in Spanish.
Now we have a smarter English speaking guy (modern CPU) who wants to do the same thing but doesn't understand jack shit about what they're talking about and even if he understood, he wouldn't know how to do it. The boss (PSX application again) is still ordering the same thing he said before, in the same way.
So the English speaking dude would first need to translate these instructions, transcribe all of that bullshit expert talk to things that he could understand and create a list of tasks in his own language and only then he could attempt to do them.
That's a lot more work so he needs to be shitloads of times faster to handle these things and he needs a good enough emulator (translator and dictionary) that he also has to use all the time to understand what he's trying to do. And obviously he's still more prone to making mistakes.
If someone was to make a new game from scratch that would be Android optimized, one that would be an exact copy of a PSX game it would run smoothly on a much slower device than a PSX game emulated on an Android device. It's called porting.
However usually a company porting its app to a different OS have easy work to do because they have the source code for the app and they just have to make changes to make it work on a different platform.
In case of PSX games it's not as easy as taking an already made game and just "making it work well" as an apk.
PSX games are not open source - developers would have to start from scratch - it would take almost as much effort as creating these PSX games did in the first place.