Can a simple DIY volume knob replace a pre-amp?

EVS makes a very simple high quality attenuator that fits directly onto amplifier inputs.

You can get a brand new Luminous Axiom for $125 brand new. I had one and it was a really good preamp. I had an Audio Research SP9 preamp...the Axiom stomped all over it.

It comes with your choice of XLR and RCA

I believe they have a money back guarrantee.

http://www.luminousaudio.com/axiom.html

There are 3 basic analog volume controls.

Potentiometers are the least expensive but capacitance and inductance can affect the sound. There are some well regarded brands such as Alps.

Attenuators use individual resistors and can be anything from simple rotary (step or ladder) to remote and semiconductor controlled (usually not for DIY). These are usually better than pots but can be difficult and expensive to assemble. The exception might be DACT units. The main drawbacks of resistance based units is reducing dynamics at lower settings and the requirement for low output impedance of the source. Placette is highly regarded due to high quality parts. The upside of passive "preamps" is incredible transparency.

The third and newest alternative is transformer volume controls (TVC). The Electraprint is the least expensive I've seen. Congratulations, if it works. There's also Sowter(IAG) or S&B (Bent Audio). These also require equipment matching but there is no loss in dynamics. I built one, as an experiment, for less than $1000 and it blew my reference quality tube preamp right off the rack. The other nice thing about TVC's is that they are completely indifferent between balanced and single-ended (autoformers excluded).

Of course, there's always Aunt Corey's but the BUF-03 has been discontinued.
http://www.stereophile.com/amplificationreviews/54/index1.html

So why do most of us have preamps? Are volume nobs better?

I also can recommend the Endler Attenuators. I bought a pair, and they're as good as anything else.

Of course, I'm not much fond of preamps for line level sources, so anytime I can run CD direct, I take advantage of it.