My bmw 540i still has a coolant leak, water pump I believe is the case but why

What is your question?

I've replaced radiator, alternator and now I belive my water pump needs replaced. Only had this car a year and purchased it w 108,000 now has 134,000. It seems never ending w coolant leaking problems. What causes the substantial coolant issue leaking problems for all these repairs? I heard head gaskets are expensive and just curious if the pattern of problems is leading me to have to replace that as well.. Any help or advice would be helpful. The water pump Pulley system making a grinding rumbling sound but it has always been leaking coolant even after replacing everything but water pump that just started today. What is the reasoning for all the leaking coolant issues?