The benefit of separate ovens and hobs is that the ovens can be placed in eye level or counter-top positions, doing away with the need to stoop. Of course, the traditional under-counter position can still be accommodated.
Slot-in cookers can still be bought, but they are generally perceived to create a disrupted line in the cabinets and also, allow food debris to fall down the gap between the appliance and its adjacent cabinets.