And why can't we always start with the proper nouns?
Could I also say Zofie se o nej stara
You can. The meaning of the sentence is changed though. Generally speaking, put the most emphasized thing at the end.
'Žofie se o něj STARÁ.' (as opposed to doing something else)
'Stará se o něj ŽOFIE.' (as opposed to someone else)
Stará se o něj Žofie why wasn't this accepted?