.
Basically, because ice floats.
Sea ice floats. Glaciers do not float; they’re on land. When glaciers melt, the water ends up in the ocean. Melting glaciers increase the volume of water in the oceans. These are undeniable facts.
The percentage increase in ocean water volume due to melting glaciers can be calculated, as can its effect on mean sea level. I have not yet seen a calculation of that percentage which is well supported by evidence.