Because nothing in it reflects light.
We can see things because light from them gets into our eyes.
If something has nothing in it to bend or bounce light, we don’t see it. This is why air is clear, and why glass is clear.
There are times when we can see these things. If the air has dust in it, we see the dust. Or if the air in one place is hotter or colder than in another place, we can see the effect of the difference because it bends the light, and things on the other side look distorted or appear to move.
If glass is perfectly clean, we might not know it is there, and walk into it. But if the glass is not smooth and flat, we can see how it distorts things we see through it, and we can tell where it is.
If the water or glass has a dye in it that absorbs some color of light, then we see all the colors but that one, and we know the glass is there. If the glass absorbs yellow light, it will look blue.
Water and most glasses actually do absorb red and yellow light more than they do blue, so if there is enough water or glass, it looks blue.