Is it really? In 35 years downtown, the area with less vacant land than anywhere, added over 200,000 jobs. Yonge-St Clair, Yonge-Eglinton are also spots that have grown. By contrast areas with plenty of vacant or underused land like SCC, ECC, and East Danforth have had disappointing growth.
Available land matters when density is a negative, e.g. for single family homes. Where density is a neutral to positive factor, such as office towers and condos, availability of land is far less important. There are a million places in Toronto where a developer could have put in 200 units far more cheaply and efficiently than Trump Tower, but there is vastly more demand for units in that part of town. If an area is desirable enough, a shortage of easy to develop land is little hindrance.
You can add all the zoning incentives and transit lines you like, but if an area is not one where people have a strong desire to live and work, it's not going to be very successful.