France and Britain made a secret arrangement called Sykes-Picot. With this agreement the middle-east was divided. I am not sure what is the main reason why they divide it to more then 10 countries. I mean they could divide it only for 2 countries, one for France and one for Britain.
The Sykes-Picot agreement did not divide the middle east into countries, but only into areas of occupation. These areas were French occupation, British occupation, Russian occupation, French protectorate, British protectorate and International zones.
These areas do not correspond with later country borders, so your conjecture that the Sykes-Picot agreement divided the middle east into the current countries is not correct. Both Syria and Lebanon was in the French protectorate, for example, and these states were created by the French, not by the Sykes-Picot agreement.