Erasing content and all settings on the iPhone does it really work or can it be recovered by anyone you sell it to

I was thinking of selling my very old iPhone 4 and I've done the necessary steps via apples advice for wiping the phone however, with hearing data recovery can easily be restored e.g. Photos,web history. I'm concerned my online banking and family photos could be recovered so why hasn't apple used the same apps as other companies to prevent data from being recoverd via hard wipe if it so believes in its security and privacy laws to protect its users and why does it give false advice stating once you've erased content and settings you cant get it back ?????