Probably is to balance the game. So you don't get high end units too quickly. I didn't really find money to be a problem once I got going. Have a few trade agreements. Sack settlements. Also just have a lots going around the sea collecting skulls and sunken treasure you could make a few thousand a turn
Tomb kings in game and in lore are all about their constructs which is fairly implemented in warhammer 2 I find. Tomb guards are decent and their best infantry but aren't that great. TK rely heavily on magic and constructs, which are awesome.