It has little to do with NIMBYism and everything to do with government interference.

First, Southern Ontario has literally millions of acres of undeveloped land that both companies and individuals are chomping at the bit to develop. The problem is, they can't; it's all zoned as Environmentally Protected. This means that you can't build on or harvest resources from the land you own. Further, even though Canada has a progressive Carbon Tax, there's no mechanism by which a land owner could monetize the living trees on their property (like a Carbon Credit) because Canada doesn't count the reduction in CO2 from trees until they're harvested, which you can't do on EP land. So the land sits there, both undeveloped and useless towards our so-called environmental committments.

Second, municipalities have been fully aware that they were running out of space for decades. They're confined by the municipal boundaries which were set some time in the last millenium. Instead of planning for this, or leaving any land untouched, they've built sprawl. Subdivisions provide the worse combination of high-maintenance and low usable space, but each lot nets the city tens of thousands of dollars in lot fees alone, before anything is even built. Instead of building 4-8-plexes with large parks and walkable amenities, they build 1/4 acre houses with tiny parks (because the yards are too small to use for anything other than a BBQ) which need cars because the low density doesn't support small, distributed retail but large centralized super-stores.

I would honestly by fine with immigration if the new people were expected to move out to the woods and build new cities, like my ancestors did, but they're literally just taking up all the low-price housing options.