why do we use default gateway IP when MAC is used?
It's perfectly true that it would make just as much sense to describe the next-hop router in terms of its link address. But just as a matter of history that's not how it was designed. In order to keep the concerns of each layer separate, it makes more sense to describe a next-hop router by IP address, and leave the lower layer to sort out any possible addressing or link to send it out of.
Remember that not all links are ethernet; indeed, most long-distance links are not ethernet of any kind. Many point-to-point technologies do not have addressing.
It is quite common to specify next hop by interface in routers:
ip route 0.0.0.0 0.0.0.0 Dialer1
ip route 172.30.20.0 255.255.255.0 Tunnel0
This is because it's quite common to have no IP address on a point-to-point link -- you really don't need any addressing at all on a point-to-point link.
And there are also "proxy ARP" situations where instead of telling a client what its router is, we lie to it and tell it that the destination IP address is to be sent to a local LAN address, rather like you're thinking about. It's not very elegant and the last time I saw it used in real life was in the 1990s. (Cisco doc)