the only two reasons I can think of are politeness(he is talking to himself) or because it might be confused with the passive(because it's potential)
This is called ら抜き言葉, 'words without ら', and while it is often used informally by younger speakers, it's non-standard and therefore can make you look sloppy if you use it in a formal environment. It can apply to any 一段 verb, so long as you can use the potential with it: いれる, 見れる, 変えれる, 食べれる etc. Note that when an 一段 ～られる is passive rather than potential you cannot remove ら.
It most likely arose because, for 五段 verbs ending in ～る, 「～れる」 is the standard potential form, plus it creates a distinction between the passive and potential forms.