Leather stiffens up with age, especially if it hasn't been cared for.
Today's leather (in cars newer than yours) is almost a plastic.
You can try slathering on a leather conditioner, then closing the car up on a warm day and let it sit. The leather may soften up (but probably won't).
You can also get them redone using modern synthetic leathers, which are surprisingly better than real leather in my opinion - softer in winter, cooler in summer, and much less exoensive, like 1/4 the cost.