Stores that say they can install a bridge in under 10 minutes while you wait (rather than having you leave the instrument for a custom-fit bridge) are probably using a product called a "self-adjusting bridge." These thick bridges have moveable feet and come in generic heights (extra-low, low, medium, and high) and require little skill to install. Just as no two trees are exactly the same, no two instruments have the exactly the same dimensions--there are no generic measurements of extra-low, low, medium, or high in violin-making.
In addition to height and thickness issues, self-adjusting bridges muffle the potential sound of the instrument. The feet of these bridges fit separately into the body of the bridge (see the image to the left). Because of this design, the transfer of sound from the strings through the bridge to the top of the instrument is slowed; when the transfer of sound is slowed, the instrument sounds muted.
When we custom-fit a bridge (using quality bridge materials), we cut the bridge feet to match the individual instrument's top shape and fit the top of the bridge, accounting for the following: neck height, fingerboard sweep/shape, nut height, etc. We also spend time customizing the bridge to bring out the best potential sound of the individual instrument--removing mass/wood from different areas of the bridge to bring out the lower strings/higher strings or adjusting to match specific tonal requests from the player.