Why not simply bring the Desmond/Altair/Ezio story to the big screen ?

They could have easily aped the AC 1 Desmond story, they were trying to in a lot of ways but they missed way to many marks, did not explain enough for new people, and did not flesh out the characters or motivations. They should have gone deeper into the whole purpose of "the creed" like how Altair is retaught and show why the apples of eden are powerful objects.

This film tried but did not try enough to make it count, they had the means but for some reason did not go all out like they should have.

"Always two there are , a master and an apprentice"

I don't know the motivation...but part of me wonders if it was budgetary? If you adapt AC1 (which would make a very easy transition to film) I'd say at least 80% of the screentime would have been in the 11th century, as opposed to this film where it was flipped and it seemed 80% of the screentime was modern day. Obviously hundreds-thousands of extras in period clothing and CGI mattes and composite shots cost a lot of money, so doing a story set largely in the modern day makes sense from a bean counters perspective, but at the same time I feel that the historical setting is one of the single biggest parts of Assassin's Creed appeal.

It's a real shame because AC1's story was a lot more straight forward and relatable than this. Altair has a clear (if simple) motivation, Desmond is easy to relate to, and DeSable and Al Muallim are very effective villains. IMO they really missed a trick by not simply adapting the game(s).

I suspect the problem is that you have too many paperclips up your nose

Because it would be unoriginal.
I think that making a new story was a good idea, it just didn't turn out very well.

it was the definition of a clusterf#ck on screen.