I don't think they dry out if not used, but very old electrolytics may need to be re-formed if they have sat for a long time. Personally I just replace electrolytics with modern parts, they are what I consider to be consumables.
Surely with cars there are lots of other things that deteriorate when sitting. Seals dry up, fuel turns to varnish, rubber parts rot, but capacitors? Maybe if you let the car sit for 50 years but by then you'll have other problems.