Because almost all sessions are inimically hostile to new people, regardless of skill level.

Not my experience. More often than not a new face with maybe some new tunes or songs is more than welcome. There can be an element of wariness, if it's a stranger to everyone there but probably less so than in many social meetings, with a newcomer who has to introduce themself. After all, there's some strange people out there.

Good manners come into it too - in a pub you'd ease yourself into a conversation, listening first, not jump in and lay down the law, and it's the same with a session.
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When it comes to keys, if someone's singing a song they should pitch it wherever it suits their voice on the night, which can vary, and it's up to anyone who is playing an accompaniment, if anyone is, to adjust to that. For instrumental tunes, any group of people playing together will develop their own understanding about what key to use, and there's no reason why that should necessarily be the same as some other session, let alone some recording someone has made. And most tunes benefit from being played in a different key from time to time to see how that works out.