You're going to need a good caretaker if you're not going to be around so that costs money. Then you have to get someone to clean up, fix stuff, do yard work, etc which is more money. Just like rental homes there will be yearly audits so you will need to fix stuff. Lastly, the calls at 2am because the toilet is broken.
We have thought of buying an apartment complex but without the support or good tenants it will be quite expensive to maintain. Put it this way, we rent houses and there are more times when we would like to kick the tenants out. I can't even imagine the process of trying to get rid of multiple people. What a headache.