They most certainly are not. Nearly a decade of planning goes into subdivisions from them being just farmland to people moving in. The first plan for Queensville, north of Newmarket, was created in 1998, with the first residents moving in next summer. Seaton had its lands initially sold by the province (or rather traded for lands in the Oak Ridges Moraine) in the early 2000's, with first residents moving in early 2017. North Oakville has been in planning for many years.
Large subdivisions almost always go through rigorous review, extensive infrastructure demand planning, multiple OMB hearings, etc. before actually getting built. The only reason they seem to "pop up" is because, as you said, there is never any news on them. Most of this planning work is not easily available, and the first easy evidence of them is when construction starts 2 years before the first occupancy.
Essentially, from day of land purchase to the first resident move in for a condo is between 5 and 10 years, a subdivision is between 7 and 15.
edit: Mike in TO beat me to it. I nominate post of the week, thats a great overview of how suburbs get built that many are completely unaware of.