Because you cannot sack for unfair reasons a contract has to be fair and legal you cannot just put any old stuff in and expect it to hold up in court.
Ie i cannot have a clause saying when you work for me you are not allowed to drink water or use the rest room. Or that you are not allowed to drive to work or interfere with things outside work that doesnt effect work like you are not allowed to go to church on Sundays because we as employers are atheists.
And if it went to tribunal it would cost you quite a lot for unfair dismissal.