If the RG had taken 10 minutes to speak with a multi-family housing developer, they would know the answer to this.

1. Construction costs are rising double digits per year.
2. Restrictive UGBs make land expensive, if you can find land that is zoned for high density development (R4). Medium density (R2/R3) doesn't work for apartments.
3. Rents in most markets, while they have risen, are still too low to justify the cost of new construction. 
4. City permit fees and SDC charges can add up to 20% to the cost of any project. 
5. Lenders have backed off financing of new multi-family projects. Those that will lend do so at low loan-to-cost ratios requiring the developer and/or investors to inject larger sums of cash into a new project. More cash for same return means a lower rate of return on capital.

Developers need to earn that evil "P" word for them to be incentivized to launch a new project and right now those projects are hard to find. Same can be said for single-family homes.