Leather needs to be maintained (oiled or polished), even if you’re not wearing them.

Leather dries out over time when left to sit and becomes brittle. It must be treated with some thing like a leather oil rub or “saddle butter”. Fake leather is similar.

When you wear them regularly from new before they dry out, the sweat and oil from your skin goes a long ways towards keeping the leather pliable.

Artificial fabrics obviously don’t have this problem.

Do you mean the upper, the sole, or some other part?

For the upper, leather needs to be taken care of. I’m not an expert on that as it’s never been worth it to me to deal with that. Fake leather has certain additives that eventually break down and then the material becomes less supple and resilient. That’s the same reason midsoles typically have a limited lifespan even without use as well.

You just have to maintain the material.

Others have already covered it, but I figure a repeat is part of why people crowd source answers.

For leather, it’s easy. You just use one of the many leather conditioning products. Oils are the norm for that. Leather will otherwise dry out and harden.

For other materials, you run into more difficulty because they tend to not to have many options.

Fake leather, only PVC has any options at all afaik. You just have to find PVC moisturizer. Other types (polyurethane is going to be the only other one you find commonly), there’s nothing at all. However, you may have some degree of success with mineral oil. And that’s not going to be much success PU pleather is a shit material for anything.

Truth is, though, the fake leather absolutely will degrade over time, no matter what you do. It’s inevitable because the plastics used just aren’t robust. They have to be flexible, so it limits how tough they can be (don’t ask me to explain why, I got this from a guy that works in plastics). You really can’t store them and expect them to last more than maybe a year or two in normal conditions.

Even the best fake leathers have a life span that’s much lower than real leather. If nothing else happens, the bond between the plastic and whatever fabric underneath it will degrade.

What’s worse is that using (wearing) the shoes isn’t going to make it any better. You still get the chemicals degrading, and now you’re adding wear and tear. So your best bet is to just wear them and save up for better shoes while they last.