ID Change with Vgs for MOSFETS
The Rds(on) specification says that if you pass a drain current of 8A, with the specified Vgs, the Rds(on) (And therefore Vds) will be as listed. The 8A is limited by external means.
If you try to pass a very high current the MOSFET will behave differently and Vds will increase faster than you’d expect from a fixed resistance. Once Vds starts to look significant compared to Vgs..