The Marine boot camp is about twice as long as that of the Army.
Army boot camp is 6 weeks long, about a month and a half. The Marines on the other hand train for just over 3 months. And from everything I have both experienced and been told, it is much more intense as well.
I heard of "Weekend Passes" and "On Base Liberties" for Army recruits, but I can tell you that as a Marine, there was no such thing. Our only "Liberty" during our entire training was for 4 hours, the Sunday before graduation.
And it shows. If you march a platoon of Marines in civies and a platoon of Soldiers in civies, anybody can spot the difference. The bearing and carriage is that distinct.
2 years ago, I went through the Army "Warrior Transition Course", where they train prior service and soldiers out of the service for more then 5 years how to be "New Soldiers". And the first day, the Drill Sergeant watched me and said "I can tell, you used to be a Marine." The way we carry ourselves is that distinct.
But as for why the 2 services are different, it has to do with a lot of things. The mission is a big part of it, and in addition the size of each.
Over my 10 years as a Marine, I would run into others I served with fairly often. I may run into a guy I served with in California in the exchange in Okinawa. Or the guy I knew in boot camp will be in another platoon in North Carolina. But in the Army, you will rarely meet other people again that are outside of your Branch.