Why does my automatic vehical allow the engine to start while enganged in drive

I have a 2000 honda passport that has n automatic transmission, it doesnt catch gears but once in a blue moon it will drop into gear.. it will slowly move and finally catch if I force rev it which is extremely bad to do but it’ll get the car moving .. my question is I just recently discovered that my car will allow me to start the car while it’s engaged in drive .. why? What part is broken? What does that mean and does it have something to do with why my transmission won’t engage? What damage could it do?