I'm curious about this as well. I don't see a down side to owning my properties outright. Before anyone jumps in let me explain. I am not trying to build an empire. My goal is to replace my 9-5 income so I can retire early. I'm 55 now so its time.
For example, I own a house, I have about 45k in it. I own it. This house easily cash flows $1,000 a month. That money just sits in an interest bearing account until another deal I can't refuse comes along and I jump on it. A little rehab and then I am cash flowing another $1,000 a month. For some reason that 1k a month has become my goal. You really don't have to do this to many times before the monthly cash flow puts a big grin on your face.
Perhaps I prefer this method because of the low stress/slow pace it allows me. If I want to spend 6-8 months doing a simple rehab that is not an issue because I have no mortgage and next to zero cost while the house is empty. I think the difference here is that different investors have different goals. I've done my work, made my money, had a career I am proud of and now to be frank. I don't want to work and BRRRR sounds exhausting. Maybe its just laziness...LOL