The electric motors can be pretty tiny. The batteries are generally the packaging problem. They're heavy and lumping them all where the engine would have been in a vehicle will have severe impacts on weight balance and handling. Distributing them is best, but requires space that vehicles need to be designed around. You can put some batteries in the engine compartment and some in the trunk to keep things neutral, but that still requires giving up storage space and requires running a high voltage line throughout the vehicle to connect the battery banks.