What's keeping them from retrying until they get a 20 is that they are just as likely to roll a 1 thus allowing the GC to activate a critical error and unleash trouble on them.
Does the GMC have unlimited action dice?
I.E.
A snoop is trying to enhance photographs taken from a digital camera on his computer. Instead of taking his time, he tries to be a showoff and starts rolling dice. On his third attempt he gets a 1, the GC activate the error by spending two action dice and as a result the agent, through bad luck and stupid mistakes, lose/corrupt all the data. The GC laughs while the agent's team mates are left to wonder how a supposed specialist in surveilance could bungle things so badly.
If you have plenty of time and face no danger, you just take a 20 and if you have any degree of competence in the skill you should succeed. There is no need to tempt the devil on a routine check, eh? Sure, you forfeit possible critical success, but so what? Reliability is a virtue.