From someone who grew up with parents that owned a hand full of rental houses and did all repair work themselves (quite competently, father is a tradesman anyway) : I am not at all surprised that individual owners tend to keep deposits at a higher rate. I don't think there's anything illegal about it : without fail, every single renter we have ever had has incurred damage to the house well beyond the security deposit. When a company owns a ton of properties, they have economies of scale- all the properties have interchangeable hardware, cabinets, etc they likely have suppliers for materials that give them more competitive rates for bulk purchases, they have workers who will do the repair work for minimum wage (whereas a single owner probably values their time in the range of 50/hr)

It makes a lot of sense, but I understand your frustration. Still not nearly as frustrating as hauling trailers full of other peoples trash and dog shit out of your own house and then having to repair thousands of dollars worth of damage to cabinets, drywall, carpet, and paint.