Why is it "jest", not "są" here?
Polish quantifiers and numerals. Dużo like most quantifiers needs specific construction.
Dużo + noun in genitive plural + verb in 3rd person singular neuter.
Dziękuje bardzo! There's always more Polish grammar to learn...
Why can one not use 'ma' here? Or is this only for negations, like in 'nie ma nikogo w domu'?
Yes, only for negations. On its own it just means "have".