I'd imagine part of that is due to the superfat content of a soap/cream - this is the unsaponified (or non-soaped) matter within the soap, hence remaining as the starting oil. I've tried out a superfat of 5% and one of 10% with some home-brew soap and that makes a huge difference in both slickness and PSF.
Further to this, the type of oil which is used makes a difference - hence why tallow is so well touted, or for vegetarian/vegan soaps, stearic acid (from palm). They're both pretty slippy in soap form and pretty slippy in their starting format, so makes for a good soap base. The rest can be stuff like shea butter/cocoa butter which are both slippy and have the nourishing qualities which people aim for.
As above, my own trials have proven this to me. Wickhams also proves this as does P&B, though I've had far more experience with the latter.