Why can we only write options from pledge margin, but not buy options?

Why don’t we have option to purchase options from the margin we get via pledging our stocks,but we can write options? When you are exposed to unlimited risk via writing?

In fact, with a pledged amount you can only transact in instruments which have margins associated with it (like futures and short options) not instruments which require you to park the entire amount (like buying stocks and going long options). This is purely from a risk management perspective since margin blocked provides a better cushion against any potential losses.