Using CE Router for Internet and VPN services

Hi, can someone explain me why service providers (Internet and/or L3 VPN services) obligate customers to use CE routers. Why they cannot configure more than /30 (in some cases /31) subnet mask on PE interface side for me? In that case I can use cheap L2 switch and use default gateway on all my PCs in the LAN, pointing to SP PE. Please describe all benefits and detriments of using more than /30 subnet on SP PE. Some good links will be very useful for me. TIA! -- AA