Can please someone explain to me how a page cache can be “far, far slower than direct I/O” in any application on any hardware? In my understanding RAM latency

Can please someone explain to me how a page cache can be “far, far slower than direct I/O” in any application on any hardware? In my understanding RAM latency and bandwidth are couple orders of magnitude faster than any storage device out there, so cache check can not be slower than a storage device access. What am I missing?