I would like to see that from different angles, and from further back. I'm also curious how the beam is secured to the posts? Are the posts just sitting on the ground?

I'm guessing there was no building permit involved here...

As to why the posts would all lean like that? I'm stumped.

I just looked at it. the posts are sitting on concrete pads and there is a 3-4” vertical rebar shaft. the top of the posts are/were attached to beam with wood scraps i guess. no metal brackets. the house is 45 yrs old. ive owned it for 30 yrs. No metal brackets at top of posts.
None has even walked on deck for 10 years. funny how life goes.
Looks like my beam is shot too.

I guess if you've never used it, you won't miss it when you tear it down (which I would do ASAP).

It can be fixed properly without demolition.

The deck wasall cedar. that had been painted grey. decking was 2x6 and joists were at 24” centers. i removed decking and added additional joists to get 12” spacing and put 1x6 treated decking at 45 deg pattern. which i hated. i did this in 1995? i planned on redoing railings for different design and was going to pull up decking this fall but i had the header issue now. so railings are mostly off now and i pulled up 3-4’ of decking over header area., its pretty much a train wreck.

The deck needs completely torn out to start from scratch - footings, posts, beams, joists, house ledger, step support, fastening types and patterns, etc. I would suggest you look up and follow, as it is pretty much a how-to guide, the code resource and sometimes mandated "American Wood Council - Prescriptive Residential Wood Deck Construction Guide" - it's free and on the internet as a *pdf.

To answer your general question of why the beam rolled over, when you place a vertical load on a member it manifests into a force that wants to rotate it. You combat that by large enough sized members that can handle that force internally and proper fastening that is not overpowered by that force.

I replaced beam and posts. was easy enough. I couldnt leave old beam as is. at least it is level now. need some metal brackets. will i replace it all? probably.