The difference is resources.
You want a fake sin lets say rating 4 nice solid one.
Doing it yourself
You have to spend time forging the SIN
You have to hack into numerous databases and insert the backstory (education, dna, biometrics, family history, medical history etc etc)
You have to link all these together in the central database without tripping any alarms.
You have to link them to your forged Sin.
Getting it through a contact
They probably have dozens of "unused" sins sitting around for people not interested in a solid match and just need to insert a few little details here and there based on your info.
They can have dozens of contacts putting in data and linking them.
They probably have backdoors into the more commonly used systems so rather than hacking in they just log in as Edwauardo J Fakenameyname and put in a record of Tom Lisa Hayes and his C average in school.
All in all its a lot quicker and easier because of those differences.
Have other people to do things rather than havnig to do it themself
Have legitimate or at least established ways into systems where you have to break in
Have a base matrix that they can probably call upon in order to supply rating 1 or 2 sins where the details don't really matter.
Now yes technically any character could do the same but its not something you can do on the side while running you'd need to make contact with people in those fields, get access to databases, get the tools and techniques to prepare it. There's a reason fixers by and large have retired and are just acting as middle men rather than running themselves.