It's not... I love Keane, and he is a world class player without a doubt and a United legend. I'm saying in general, global football view and comparisons of players over time - not just United fan POV. Casemiro has achieved more than Keane did (without question), while as players I would say they are very similar in the level they produced at their very best.
It's like how United fans will Scholes into the Xavi/Pirlo/Scholes argument, when most global football fans don't look at Scholes as part of it. And Scholes is arguably my favourite ever player. But the general view across most football fans, is they wouldn't mention Scholes in those discussions (I personally wouldn't include Pirlo on the same tier as Xavi but on a similar to Scholes, but again, most non United supporters wouldn't).