In practical BJTs, why can't Vce become 0 when the transistor is saturated?
Think about it - if you are forward biasing the base-emitter junction this means that the base is several hundred mV above the emitter for an NPN example. If you want the collector to be close to the emitter voltage (i.e. at 0 volts) then you are also starting to forward bias the base-collector region and, in-effect, the base keeps the collector potential from ever reaching 0 volts.

This is a simplified answer that suggests that the OP considers the NPN transistor as two back-to-back diodes for the sake of understanding how perfect saturation is impossible.