Why do Arabs find it very difficult to learn an American or British accent when speaking English?

i think because , they don't study the way of how the sounds are pronounce, because there are some sounds in English language which are not exist in arabic language Like /P/, /V/, so if they focus on the pronounciation, i beleive that they can first make a clear distinction between american and english accent, and that can help them also to learn one of these accents or Both of them 

I think because there are some sounds that do not exist in the arabic

It is not difficult to learn any accent.We should just listen and this will help a lot.

This is mainly because of MTI. MTI or Mother Tongue Influence plays a big role when trying to learn or speak other languages. A big example can be the way Americans and Britishers and Australians speak the word "Often".

"Nothing is impossibles" if they learn they can be speak as similar. but they want their own language to be preserve. we all are equal and Arabic can be if they learn.

As per my personal experience in Saudi arabia, the people there dont encourage english language to that extend and they only speaks arabic so this makes other nationalities harder to make conversation with them and they dont give themself a chance to learn english in their day to day activities.

Arab language and English language are so different. That's why they are expecting this kind of problems.

missing of alphabetical sounds like 'p'

They can't simply because it's biology. Accents are acquired and not taught. The best age to speak any language without an L2 accent is before twelve. After that age, the elasticity of the brain, as scientists define it, becomes rigid and less adaptable which shows in how we speak. You can still be fluent more than L1 speakers but the accent will always be there. 

I think beacause there are some sounds that do not exist in the Arabic language like the /p/ and /v/which are not easy for them to pronounce.

ARABS FINDS LEARNING ENGLISH HARD BECAUSE THEIR GRAMMAR & SENTENCE FORMATION IS COMPLETELY OPPOSITE.THEY CAN LEARN ENGLISH LETTERS EASILY BUT TO FORM A SENTENCE WITH THAT IS CHALLENGING FOR THEM. EVEN THEY HAVE THEIR OWN NUMBERS IN THEIR LANGUAGE.