There are many reasons why credit cards are safer kept in your wallet. These reasons will relate both to the security and safe-keeping of the card as well as ensuring the card is less likely to know inadvertent damage.
Credit cards are safer kept in your wallet if you are to minimise the risk of accidental loss. Keeping a credit card loose in such as a shirt pocket or any other pocket can very easily lead to it slipping out of same unnoticed and being lost. This is because they are so slight and light that you are unlikely to notice their absence until it is too late. Loss of a credit card in this way can at best lead to the inconvenience of having to cancel the card and request a replacement. It can at worst lead to subsequent fraudulent use of the card by some unscrupulous person finding it and considerably greater inconvenience and even financial loss for the card holder.
Credit cards are safer kept in your wallet to prevent them from being misplaced within your home or office. It is so easy to remove a credit card from a pocket or purse, sit it down somewhere and be later unable to recall where. They take up such little space that they can easily be covered up by something else or slip down the edge of such as a chair cushion. In a worst case scenario, they could easily be picked up with something else and discarded in error.
Credit cards are safer kept in your wallet to prevent theft by such as pick-pocketing. Although a wallet can of course be stolen in this way, the increased bulk and weight of a wallet means it is far more likely that you will notice someone attempting to steal it in this fashion. It takes a far more skilled thief to steal a bulky wallet than it does to steal a slim line credit card.
Credit cards are safer kept in your wallet in order to prevent accidental damage to the card itself. It is so easy to forget a credit card in such as a hip pocket before sitting down. This can cause damage to the magnetic strip on the card through buckling and mean that the card can not be read the next time you attempt to put it in an ATM or any other form of card reader device. This could of course lead to considerable inconvenience and even embarrassment at being unable to pay for goods or services provided.
All round, it can therefore be seen that credit cards are safer kept in your wallet for a number of reasons and that there are few, if any, justifiable reasons for not storing them in this fashion.