I've made movies and software. I'm extrapolating from movie experience to imagine I know something about marketing and advertising.

I'd feel more comfortable presenting a fixed project quote for a creative project than a software project, because in the case of a creative project I can choose to dedicate a fixed amount of time, and have the result - to be judged subjectively - that I was able to produce in that time.

In the case of software, I would need to keep working until it objectively meets requirements. The quality of software I deliver doesn't degrade smoothly under time pressure, it falls off a cliff.