I used to work in a bank that merged with another and the amount of inefficiencies, double teams/management and general disgruntlement was staggering. I left and so did most of the ones capable of finding new jobs. The ones that stayed behind were not the highest performers.

Another one is from macro economics. When two companies compete they tend to be very efficient but when they merge they have no more incentives to compete to attract customers. The result is higher prices to customers and lower effectiveness. Imagine what would happen if Intel bough AMD.

But the main issue is that I want to manage my investments myself and not let the companies manage it for me by merging.