Google is in the software business. Some ISP's are in the software business, like AOL or NetZero, and some are also invested in physical plant like Comcast and Verizon.
What Google is talking about is building physical plant which is a whole different business than software. They are going to need installers to run fiber into homes, technicians to fix problems, customer service reps to answer phone calls from customers having problems, etc.
They are wanting to become another Comcast and that's a big departure for Google.
BTW, I could give you a much longer list of companies that have failed when they got away from their core business.