With email, you have to explicitly select recipients. You're essentially saying "here are my photos, I think they are relevant to you specifically". The onus is on the sender to figure out what's good for the receiver, and it's considered rude to send many frivolous or irrelevant messages. Think of how you grumble when you unsubscribe from some company's mailing list - this is an example of this social norm.
With Facebook, you say "my photos are here, anyone may look at them if they want". Figuring out whether the content is relevant is now the job of the receiver, not the sender. Facebook's UI is well aligned with this role: unlike email, where you must explicitly download attachments and mark messages as read, on Fb image previews are displayed inline and to never see a message again you must only scroll past it. Advertisements aren't considered intrusive, it's just content from another source that is (in theory) just as easy to ignore. The social dynamic is very different, and so it's used to send a different sort of message than email.
Edit: ok maybe around 40, not 75. But point still stands, it’s indirect communication rather than remembering about that cousin of an auntie’s grandparent’s nephew’s sister-in-law that I met ten years ago.
The functionality is no different to iMessage. I don’t know why this happens. It just does.
Don't you have your friends in a private contacts list/address book? Isn't it trivial to send a photo attached to an email? What is the problem?
Note: I do not and never have used facebook, genuinely don't understand what the deal is here.