Space heaters come within a wide wide range. You can get yourself a small radiant heater that just large enough to warm your legs or you can get an electric heater seems like a fire.
The other reason may be the they are afraid any particular ventilation to manage as is essential in other combustion house appliances. As their names suggest, these heaters need electricity as fuel.
There much more expensive and more companies also as people jumping by the band wagon of portable heater pros and cons infrared heaters as they realize the many benefits how the heaters offers.