Why can't one see tidal effects in a glass of water?

A very sensitive device would be required to measure the minuscule change in the water depth along the glass walls, because the differences in the strength of the gravitational field between each side of the glass are essentially zero. Because of this the force exerted on the glass is the same, but due to the small volume of water in a glass as opposed to an ocean it would be very hard to measure any change. However, the difference in the strength of the gravitational field between the side of the Earth closest to the Moon and furthest to the Moon is enough to pull water more towards the side close to the Moon.

It's also worth nothing that the surface tension, at these small sizes, will have a much more significant effect than tidal forces, further hindering the measurements.

Because the differences in the strength of the gravitational field between each side of the glass are essentially 0. However, the difference in the strength of the gravitational field between the side of the Earth closest to the Moon and furthest to the Moon is enough to pull water more towards the side close to the Moon, causing high tide for that part of the world.

If you had two glasses of water, connected by a tube, sufficiently far apart, you would see a tide, because the force of gravity due to the moon would be different at the two locations: