Just to answer the second part of the question, you do not want an amplifier that has too wide a bandwidth, mainly to lower the complexity (and hence also the cost) of the amplifier.
All users of radio spectrum (including radio amateurs) must not cause any interference to other users of the radio spectrum, and that means that out-of-band transmissions must be kept to an absolute minimum.
Since all radios generate some out-of-band signals, there is usually some degree of filtering to make sure that those signals are kept to a minimum. More expensive radios usually have much more filtering than cheaper models.
An amplifier will amplify all signals within its passband, and so if you had a wideband amplifier that amplifies all signals from Top Band (around 1.8MHz) to the 70cm band (around 430MHz), it would also amplify any spurious signals coming out of the transmitter. (And this is before we talk about parasitic oscillations within such a wideband amplifier.)
For this reason, all good amplifiers that support multiple frequencies have a switched passband, where each band is selected individually (usually manually, but solid-state amplifiers have started to include automatic band switching) and there are good filters on the input and output to keep out-of-band signals to a minimum.
This obviously adds to the cost and complexity of an amplifier, and to cover a very large number of bands would at some point become prohibitively complicated or expensive.
A better alternative is to use less expensive components, and design an amplifier for a specific band or set of bands (e.g. HF, VHF or UHF, or perhaps a VHF/UHF amplifier that does not cover HF).