The ISS's orbit is not particularly stable and it needs regular reboosts which leaves everything else scheduled with this in mind.

Providing food and other necessary resources for a long stay would not be especially difficult, but there's no need for it, and that leaves more volume to store other stuff.

The ISS, despite being a 'station', is very small with a pressurized volume of about 900m^3, about 100m^3 less than Starship. The linked capsule design also means the real usable volume is substantially less.