1. I believe* the mobs are attracted to sound and light.
Bee's make a humming noise so at a guess they are attracted to them because of that.
Same as they get attracted to portals because of sound/light.

2. Honestly, i've built "tent" houses (2 high/45 degree roof) around my larger bases to trap greylings because of this behaviour. Specifically near the edges of my ports where they swim around.

If you really want to protect your area build workshops and leave no gaps between their radius (over lap the circles slightly), this should prevent major enemy spawning within your base (except for events), Then you build wards at the perimeter of your base keeping in mind to not leave gaps between their radius's.

Also, always trench around your base. It traps enemies and redirects them to choke points.

This is the trench i dug around my city. Placed spike walls on the outer walls of the trench with wards all around the perimeter so they barely if ever get damaged.
https://steamcommunity.com/sharedfiles/filedetails/?id=2513393674

I personally start in the water, dig as low as possible then work my way from there around. If you start your trench in water and its deep enough, it will have water in it, this works even with small ponds.