Playing favorites

Okay so I need to get on my soapbox a little and rant.
Why do some parents play favorites? I mean obviously favor one child over the other? Why do that to children? My husband is a victim of this from his mom. My mother in law favors my brother in law over my husband and always has. Some examples:
**All of this is material things pretty much but still**
1. Take today...Zach's birthday....my husbands birthday is a couple of weeks before Christmas...so his stuff gets kinda blended together (which also bothers me). Zach got just as much stuff as Wes for Christmas and then today my mother in law is buying him a Wii for his b'day.
2. Wes got told he could only go to Northwest Shoals for college. For those of you that don't know Northwest Shoals is a community college in Muscle Shoals, which means that Wes had to stay at home to go to college. He wanted to go to UAH for engineering but his mom absolutely refused. Well Zach got to choose anywhere he wanted to go...and that is anywhere in the US not just Alabama...and as most of you know he goes to Auburn.
3. For graduation from High School Wes didn't get anything. For graduation from college, which came two weeks before Zach's high school graduation, he got a laptop, just a standard Toshiba. For Zach's high school graduation he got a Macbook, money, and a new Honda Civic.
4. Wes has never ever had a new car...his dad sells used vehicles on the side and Wes has always gotten whatever his dad had on hand, and then if hid dad sold it he just gave him the next thing there was. Heck Wes has never owned a car that has less than 100,000 miles on it.
5. I have been hearing from day one from my mother in law how Zach is so much more responsible, and book smart than Wes. Then she thinks that she is doing good by saying but Wes is good with mechanical things.
News Flash to my mother in law....your first born is just as smart as your second born....but you really screwed with his self esteem so much that he just didn't try anymore. And Wes is one of the most responsible people I know. Now don't get me wrong...I don't blame Zach for this....it's not his fault....it's my mother in laws. She doesn't think that she plays favorites, she has looked me in the face and said seriously that she treats her children the exact same. What a crock of bull. She is lucky that she didn't make her sons enemies to each other over this. Oh yeah I forgot to mention that when they were kids Wes was always the one who got into trouble if they fought, not Zach even if he started it. My mother in law's reason? Wes was bigger than Zach and he could hurt him. So what did she think would happen to Wes when Zach threw a GLASS pepsi bottle at his head? Yet again Mother in law says Well Wes must have did something to antagonize him.
I wish this were all me overreacting. You know what's so sad. Wes has gotten so used to this. He actually is pretty big person about it saying that he doesn't care. He might not, he might be used to it, but I am not! It's not about the material things to me. But I do think you should be as fair as possible with your children. I mean I am the baby of my family and I can honestly say my parents treated me and Chris the same. They spoiled us both.
Stepping off my soapbox now. Thanks for letting me rant.