Why amps, pre-amps, integrated amps???

Each separate component has its own sound, and capabilities sonically, and mechanically (flexibility).

An integrated, or receiver, can be good for someone who likes the sound of everything ( each individual section)and flexibility it does.

I'll try to keep it simple with the three you mentioned:

The preamp has its own sound, certain types of needed inputs for everything you want to connect to it. A preamp is needed whether it's separate, or part of the integrated, or receiver.

The power amp section has its own sound, and output power limits to drive different types of speakers, or more than one pair if needed. It can be picked out to drive very demanding speakers an integrated, or receiver can't. In some cases, a power amp may not need a preamp, but it would need a volume control, and possibly a selector for multiple inputs like a preamp would have.

Every tuner has its own sound, and signal performance, and other features whether presets, or other preferred items.

Then you can get specialized. If you need a super good tuner for distant stations, or live in an area where the station you like has another one interfering with it. The separate tuner may out perform a receivers tuner, if this would be the case.

For the amplifier section. A separate amp can be picked out
to drive very demanding speakers, a receiver or integrated can't.

Then you could get more extreme (or specialized/custom) and mix tubes and solid state for sonic reasons, plus more variable capabilities and limits each type has.

You could use a strong tuner for a distant station, and low powered integrated, if you don't need the power for loud listening, or certain types of demanding speakers, for one example of using separates.

The list could almost be limitless for our personal preferences for all of each of individual component we choose limits for our preference.

So, if a receiver does everything to your satisfaction, you shouldn't need more. The same for an integrated if you don't need a tuner.

If you have a company known for great amps, another known for great pre amps, yet others building better tuners than others and on down the line then seperates allow you to go in whatever direction you wish. The reciever or integrated locks you in but folks with seperates can get the sound, features or even cosmetics in any combination they wish.

If your speakers need much power, you need a bigger amp(s) than an integrated can accomodate. It is difficult to find an integrated which equals the performance of separates. They necessarily involve compromises due to size constraints. The arguments in their favor are that they potentially allow one to eliminate a whole bunch of expensive cables, and usually take up less space. However, some "integrateds" have a separate power supply, which negates the size advantage.

You see an awful lot of "reason for sale - downsizing" ads on the Gon these days, as the boomers age and move into smaller places. That usually means getting an integrated to replace separates.

"It is difficult to find an integrated which equals the performance of separates."

I would tend to disagree with this as a whole-sale blanket statement.

Integrateds are often overlooked as the 'value' component.

For a given price point, integrated amps can be the better value and performer. For a given price point, separates involve additional costs for separate chassis, power supplies, interconnects, and even power cords.

Take the same amount of money and you could easily step into a 'better' integrated over separates.

So, it depends.....

Why not.