They are taxed, but I think they could be taxed more and better. Specifically, we should implement a land value tax (LVT).

As for why LVT? In short, it's just a really good tax. Progressive, widely regarded by economists as "the perfect tax", incentivizes efficient use of land, discourages speculation and rent-seeking, economically efficient, and hard to evade. Plus, critically regarding landlords, land value taxes can't be passed on to tenants, both in economic theory and in observed practice.

As for the difference between LVT and property taxes? This video explains well how property taxes enable land speculation and disincentivize housing development, and how replacing them with land value taxes would alleviate these issues.

Further, even places (such as the Australian Capital Territory) that have implemented quite milquetoast LVTs have seen positive impacts on housing affordability:

It reveals that much of the anticipated future tax obligations appear to have been already capitalised into lower land prices. Additionally, the tax transition may have also deterred speculative buyers from the housing market, adding even further to the recent pattern of low and stable property prices in the Territory. Because of the price effect of the land tax, a typical new home buyer in the Territory will save between $1,000 and $2,200 per year on mortgage repayments.