Far too often, antivirus products follow the "cable television" market strategy:
"Yes, we know you already pay us for a subscription, but we can get so much more out of you by forcing you to see all kinds of shit you really don't want, including adverts for all our other services."
And, in the case of free antivirus, this too:
"We can see that you really dont want our full package, otherwise you would have bought it instead of opting for the free version-- but we feel compelled to try to upsell you each and every possible opportunity, and wont relent at all. We will even be really obnoxious with your notification area, and make your system play audio adverts, because that's how much we really want you to have a subscription (but see the prior market strategy-- we wont let up on the ads even if you do!)"
They invest tons of resources (both computational and time-wise) into making needlessly flashy UIs with big colorful buttons, and scary "CSI: Miami"-esque dialogs, when really--- the part that really matters-- how well they can trap execution events without bogging the system down-- seems to get nearly no love, and appears to get shittier and shittier.
Then you have Windows Defender. It's so plain, you instinctively ignore its presence. Excepting on older XP systems, (where there was a CPU utilization bug), it runs with a very modest system footprint. It does not constantly vomit spam into your system tray, and does not try to milk you for additional service agreements, or to switch to a paid version. It behaves itself very well.
If Avast or AVG behaved like that, instead of trying to be garishly tawdry and whorishly self-promoting like prostitutes, and reduced their system resource consumption habbits accordingly, they would win hands down.
But no, fleecing idiots is much more profitable.