Muriel to Atalanta for near 20m
Well, I'll say it again
Everton - Tosun 27m
Bournemouth - Solanke 20m
Aston Villa - Wesley 25m
Brighton - Maupay 23m
Newcastle - Joelinton 40m
Several transfers in Italian/German leagues with random strikers going for 20-30m
So based off that, and given Calvert Lewin's record is every bit as good as the list above in the premiership (actually better than those above) and rated by many pundits AND he is English. I would say in the premiership alone: Watford, Norwich, Villa, Bournemouth, Burnley, Brighton, Newcastle, Crystal Palace and Sheffield. 20m isn’t reflective of the market, at all. Selling a 22 y/o English striker who is your equal leading goal scorer in all comps isn't good business - its irrational thinking. Which is why - no matter how patheticly this club is currently being run; it just won't happen.
As to why - I think that answer is self-explanatory. Each manager inclusive of the caretaker has rated the DCL. The club has no interest in selling him and the player has no interest in leaving – so what makes you think clubs would be lining up for him.
If he was actually available, then sure the listed clubs would be lining up, perhaps even clubs in Germany.