I know that ‘therefore’ is the way it’s normally spelled, but I always thought it was goofy: all the other there-compounds (which are cognate to German’s da-com

I know that ‘therefore’ is the way it’s normally spelled, but I always thought it was goofy: all the other there-compounds (which are cognate to German’s da-compounds) & where-compounds (cognate to German’s wo-compounds) don’t change the spelling of their prepositions: thereafter, thereon, thereat, whereafter, whereon, whereat &c. I also don’t understand why modern English doesn’t take advantage of them like modern German does.