Because they're two different businesses and before they come together they would have to come to a compromise, of how they are going to split the cost, power, film time and who to promote as well as story lines.
That's a lot to get to agree on... but some day it might happen, just like they did in the comics but that might be a long way off now.
Also while both are doing very well in the box office, then why do a combo that they'll have to split?... a combo probably won't come unless they start to struggle to sell tickets at some point
Because Batman would get mad and kill all of the other characters, blaming them for destroying a successful business.