Jisho

no idea about the し (indicates reason??) but I think that the も is used here as an emphatic marker as in そんな簡単なこともできないのか。I think I ve seen that in A Dictionary of Advanced Japanese Grammar somewhere around pg. 65-67. 血は水より濃い is also correct but less...emotional?

I've been answered on jref that 濃し is Classical Japanese which may be seen in proverbs, maxims, etc. And も for emphasizing. Thanks for the attention.

Correct, that し is often seen in old proverbs where you might expect an adjective. Another example is 良薬は口に苦し (Good medicine is bitter in the mouth)

Adding も to より gives it more impact.

Nice answer by Higashida !