Those systems are possible, but they add weight and complexity to launch vehicles that strive to minimize weight and complexity. There's a cost to build and launch such a system, there's risk of failure of that system itself, and there's risk that that system could be the cause of catastrophic failure of the entire launch vehicle, especially if an ejection system uses any sort of explosive device to achieve ejection. Simply put, those who design and launch satellites have determined it is better to risk the loss of a satellite here and there than to try to reduce that risk with an ejection sytem.