There are a few reasons for this. The first reason is that the contracts that are copied may not be intended to guard against or provide for the same concerns you may wish to guard against or provide for and the wording copied may be ill suited for your purposes. Another reason is that sometimes the contracts being copied are poorly drafted and by incorporating these poorly drafted provisions into your contracts, you may only aggravate matters. The end result is the Frankenstein version of whatever you are trying to draft and headaches down the line.
It is not enough to have language in a document that looks like the right type of legalese for your needs. The contracts you use should clearly record the agreement that has been reached between the parties and provide for the concerns and eventualities that may be specific to your arrangement. If your contract is not clear and does not provide for your circumstances then it is a waste of paper and ink. A poorly drafted contract will, in most cases, lead to uncertainty and even litigation which will be expensive. It is worth your while to have your preferred attorney prepare (or at least review) the contract/s you intend using before you pass it on to the other party/ies. If need be, spend a little bit of money making the contract the best it can be. You may just avoid that uncertainty and even more expensive litigation if and when your business relationship sours.