Because corrosion must have produced a wide-enough gap between major moving parts of the pump to allow the leakage. This doesn't mean the pump won't work, it will but you will have to keep filling/topping-up the water level...and more frequently as time goes on. I experienced the same in my old Merc. The funny thing is that while the pump is mostly aluminium (at least on the outside), their are iron components on the inside that will suffer from corrosion especially if the water in the system is...water. For longevity, the coolant should not be water alone but rather a mixture of anti-corrosion/anti-freeze liquid mixed with water in a ratio that caters for the climate your car operates in.

What else did you expect to leak from the water pump?

A replacement is the only answer, I'm afraid.

It's leaking "water"? Well, if you only had water in the cooling system, that's why it's leaking. There were no lubricants/conditioners to keep the seals intact. You need to have the pump replaced and use COOLANT in the cooling system.

If it's actually leaking coolant/antifreeze, well duh, genius, the pump is shot. Have it replaced. Use ONLY G-05 (gold) or genuine Mercedes (blue) coolant in a Mercedes-Benz.

Because the water pump is worn out and the seal has started to leak. Get it replaced ASAP as it can get worse very quickly.

Because the pump shaft bearing seal is worn out and fluid is coming out of the pump shaft weep hole.

It is time to replace it.