My first thoughts are a form that is up to 20 pages long is typically skimmed first before filling out, and I can do it anywhere. Once its online I am forced to use a computer with at least dual 24inch monitors so I can review several pages side by side.
Its similar to building a shed, which I just did. I reviewed a 20 page paper doc first so I understood what was going to be required of me first before jumping in. This had me prepared in advanced for the next step.
How many times have you filled out an online medical claim form thinking it was just going to be a few entries only to discover that there was going to be a heck of a lot of ground work required before you could complete.
Once its online I can only see one page at a time, I cannot review that page 10 is going to require me to have proof of title ownership or other identification that I have to dig up, I can't see that information on page five relates to something on page seven. I can only see one page at a time and have no idea what is next. Every page becomes a possible unexpected context switch, and we know how context switching at work takes a toll on project completion time.
Plus if it is that long and I am a domain expert, I probably know that only two pages are required from me, the supporting documents I need and the rest are simply sign here, sign here. Its similar to a life insurance application. My agent is the proxy expert and has already filled the important parts, crossed out what does not apply and all I have to do is sign on page 3, 10 and 15.