Agents for Work Authorization

Hi,

I'm a software engineer from south-east asia, currently in USA as a student.
I have 3 years of experience in .net.

In US there are some agents who process H1 visas (so legally we would be working under their company) and outsource the employees to other companies.
Does anyone know of such an agent or a software company in canada that's willing to sponsor for work authorization?

I've got so many offers, but once I tell them that I don't have the work authorization they don't want to proceed any further. I can't understand why companies are so reluctant (since from what I understand work authorization for canada can be obtained within 10 - 45 days (not like H1 in USA).