|why converting dec to bin can't use each dec correspond 4 digits binary like hex |
Well, the answer is it can. This encoding is called Binary Coded Decimal (BCD).
It was apparently used by some early computers, but it wastes storage and goes against the way computers work, as Chervil has said. So all computers now use hexadecimal (I guess there are exceptions, but not in any PC, Mac, Android-device, iPhone, ...).
Though, as Chervil has also said, hexadecimal is just a convenience. Computers actually use binary as they are, after all, just a huge load of on-off switches. Using hexadecimal allows humans to write down the values is a more readable form without getting the computers to do more work.
But BCD requires the computer to do more work.
So, it's not that it cannot be done. But that it is less efficient so people chose to do otherwise (i.e. stick to standard binary encoding).