Most umbrellas face outward only
The standard grant pays what you owe somebody else. If a driver with minimum limits causes you a catastrophic injury, your own underlying underinsured motorist cover responds and then stops, and a plain umbrella sitting above your liability does nothing about it. The shortfall is yours.
Excess UM and UIM has to be requested
Many carriers offer it as an option on the umbrella and it is routinely left off, sometimes because nobody mentioned it. It extends your own protection to the umbrella limit against drivers who cannot pay, which in a state with low minimum limits is a real and common exposure.
The worst outcome is being hurt by somebody with nothing
An at-fault claim against you is covered by your liability. A serious injury caused to you by an uninsured driver is only covered by what you bought for yourself. Households insure the first exhaustively and the second by accident.