Ok, now that you mention it: I think the difference is that (at least in my region) the PlayStation was sold with a memory card included. Standalone memory cards for it were cheap. N64 came without a memory pack and they were more expensive.

IIRC PS also had a more granular slot size (eg gran turismo takes up 1 slot while final fantasy takes up 3 slots) while on the N64 it was large and fixed (each game takes up one large slot even if that slot doesn’t use up all the data).

In hindsight that has me wondering why they didn’t go for dynamic slot size 🤔. Maybe because a save file could grow over time and they wanted to ensure that you could always overwrite/update?