Getting a professionally produced voiceover demo is an investment, but a ton of work goes into it. I purchase different music tracks and sound effects for each demo I create. It's a meticulous process. Like an artist working on a painting. They'll sit and stare at a color for 15-minutes and then change it! Sometimes they'll even start over multiple times. That's what I tend to do with audio clips on a demo. A professional photographer does the same thing with headshots for on-camera talent. A lot of post-production work goes into making sure the photo is talent agency-ready.
That is my responsibility as a demo producer as well. Make sure the demo is talent agency-ready! The production of a demo is a process that should never be rushed. It can take several weeks to complete a professionally produced demo due to the demanding work that goes into it.