Ajax is being slept on because Eredivisie is meh.
They have a European Cup 3-peat and also that dominant 1995 CL winning side...they've trashed/beaten the likes of Bayern, Inter, Juve, Milan in European cup/ CL semis and final.
They've produced an iconic style of football that preceded modern Barcelona. They've produced a long list of absolute legends (Cruyff, MVB, Rijkaard etc.).
Their contributions to football are way more than those perennial bottlers Arsenal and Bayern's servant Dortmund. Chelsea can be argued a tad bit as they do have 2 CLs but I still don't think Chelsea's contributions to football in terms of accolades and influence surpass Ajax.
Exact same reasoning for Benfica. Benfica has played in 7 CL finals. That's more than Chelsea (3) + Dortmund (2) + Arsenal (1) combined.