because a grinding disc on an angle grinder is meant to grind. When you are removing paint you are not grinding, you are sanding...you only want to remove paint, not the metal. A flap disc on an angle grinder removes metal and fast. I use an 80 grit flap disc on an angle grinder for grinding down welds only. Do not use this to remove paint.
I have a 60 gallon air compressor with a 3.7 hp motor and it still has to work to keep up with my DA. Prob a mix between a cheap DA and using the wrong DA for the job, but 80 grit on a DA gets me nowhere quickly. Now I'm giving this to you from an at-home DIY approach. Professional body shop guys have good air compressors and good DA's. My preferred tool for stripping paint is a wire wheel or cup on a 4.5" angle grinder. The benefits are it removes paint, primer, rust, and filler faster than any method I've seen. But it covers a much smaller area than a good DA so it may be a wash. It can also get tiring on the hands from the weight and constant vibration, not to mention it throws the wires at you from the cup and they get stuck in your close.
It's still the most used tool in my arsenal for paint and rust removal. I also have an electric orbit sander, pneumatic DA, 80 grit sandpaper sheets and blocks, and a panel spot blaster, and I employ use of all of these for paint and rust removal.