Some options are heavily traded while similar ones have almost no activity. When you try to implement a strategy you discover the quoted price is not the real price and the arbitrage disappears. Opportunities are not obvious. The mispricing will probably develop between options that are not expiring on the same date and probably between puts and calls. It requires a computer with live pricing and very sophisticated software to spot them.
Options are very difficult to value accurately.
If you do not have the sophistication to price them properly you can not be sure you have a mispricing. You can not trade for free. For reasons that will always confound investors sometimes the mispricings get worse before they get better. If you are highly leveraged in the transaction you can lose all your money before you get a chance to make any. There are people who play this game seriously.