But then you'd be introducing a new letter to take the place of a consonant pair, where the individual parts already exist.
It's more efficient to have the two letters which can be either separate or connected, than to have 2 letters plus a third letter to represent their combination.
Serbian and other Cyrillic languages pretty much all use phonetic alphabets, hence the use of things like e.g. 'ch' 'sh' 'shch' 'ts' -- CBF putting the proper Cyrillic in -- instead of just putting the corresponding letters together in a word.
Those languages also lack the individual letter 'h' hence multiple letters where English just pairs up already-existing ones. Romance alphabets use letters more efficiently, hence needing fewer of them.