you may or may not think that obesity is a disease, but there are diseases that lead to obesity.
There are other things also:
-debilitating injuries that make it difficult or impossible to exercise for long periods of time.
-horrible things that have happened in people's lives that lead to depression
-medications for illnesses that have a side effect of excessive weight gain
examples:
1.) let's take Hank from Breaking Bad. He had terrible injuries. He eventually learned to walk again, but being bed ridden for that long of a time period can cause quite a bit of weight gain. 
2.) I have a friend whose fiance and dad died within the same month.  She was once the captain of our high school basketball team, extremely athletic, social and outgoing.  She was in great shape in college as well, until this terrible time in her life. She became horribly depressed-- barely left her room and took a break from school.  She struggled with getting past these events and within one year had gained over 100 pounds.  Probably a subconscious desire to put up a barrier between herself and the world.  Eventually, with therapy, she was able to recover and move on with her life. She lost the weight and is doing great now.  Who the fuck would you be to judge or criticise her for being overweight and going to the grocery store or the gas station or something?  Different people are dealing with different issues. 
3.) some thyroid medications can make you gain a lot of weight, as can other meds.
But it really boils down to this:
Everyone has things they're self conscious about.  Just because far people wear their insecurities on the outside doesn't mean you have the right to put them down.  If they're not affecting you in some negative way then what is the big deal? I wish you people were forced to wear giant poster boards taped to the front of your shirts, with your insecurities listed in sharpie pen: "doesn't spell well.  Was arrested for shoplifting. Has a small penis."  See how it would feel to be judged by every single person you came into contact with-- then you'd know what it's like to live just ONE DAY in the shoes of someone who is obese.
You think people PREFER wheel chairs? get real.
It's also not true that every fat person partakes in no exercise. My dad has a friend who is about 50-75 pounds overweight, and he runs everyday on his lunch break--has been for years-- just pretty much stays the same weight.