A 100W amp for vhf/uhf in many cases, isn't that practical. HTs could use the power, but they often have weak receivers that can't be fixed.
Mobile rigs have better receivers, but put out too much power for amplifiers and turning down the power may not be easy.
A low power setting may have momentary glitches or power spikes that will damage expensive power transistors. Fixing that can be a challenge.