Because managing a mailing list for each individual report is bullshit work, a waste of time that can and should be avoided. The managers in question can either set up and manage their own mailing lists, or log into a dashboard that remembers their custom view settings. They can even have the dashboard mail a copy once a week, but they have to check the boxes themselves. It's pull versus push reports.
The first option is never ever going to happen, no manager can be bothered to maintain mailing lists. The second option for the custom dashboard is the best solution, because it gives the managers the customized views they want, without the time-wasting activity of maintaining mailing lists and custom reports. It's a matter of 30 minutes spent once for the manager to set up a dashboard filter, compared to hours wasted every week maintaining mailing lists and custom reports. If they can't figure that out, they're not fit to manage other people.
I've been doing monitoring and reporting for the last 7 years, I know all of this from experience. Report mailing lists turn into uncontrollable messes quick, but a simple webpage where people can choose for themselves exactly which info they want and/or see them on a dashboard is the only sensible solution. Mailed reports are fine, as long as nobody has to waste time managing the mailing lists.