For example I would be very happy to interview for Google 2 years ago and endure the process. Today I could sit with them for an hour and talk about stuff while sipping coffee. I have more opportunities than I can pursue anyway so spending hours solving mazes like some kind of a lab rat is not something I am going to spend my time on.
I am not a top expert in what I do but I can write and ship code. People like me (and especially ones better than me) will just not show up for your "process". Either you pursue them or you won't meet them. You will only get people for whom you are so attractive an option that they are willing to donate several hours or a whole day to have a low % shot. Those will not be the best programmers as the idea of paying to get evaluated is not something people with options entertain.