the Um (or Air) interface is open in order to allow mobile equipments from different brands to be used with BTS from other brands.
layer 4…7 are applications layer. They are not managed by the mobile network (not the BSS at least). Those are protocols used by distant servers, etc. For instance, HTTP / TCP-IP, those are higher layers.
Layer 1, 2 and 3 are “transport” layer. That’s what the BSS does : it transports data from point A to point B. No need for other layers.
the Abis interface is closed so that a BTS from brand “A” can work only with a BSC of the same brand. It gives opportunity to vendors to develop proprietary features, which are not (yet) defined in 3GPP.