Lots in existing neighborhoods are valuable to the extent a house built there would be worth more than its construction costs. E.g. if houses in the area in perfect condition are selling for $100/sf and new house construction costs $150/sf, the lot is not worth much because it is an option to build a house that will be instantly worth much less than the cost of construction. Also, anyone moving into the area would just buy one of the many $100/sf existing houses for sale, even if they have to do a remodel costing another $30/sf. Other than that, the value lies in the option to sell it to a neighbor who wants to built a garage or garden, or a crazy person who wants "everything custom!" Also, there is constant competition in the market for lots. They can stay on the market for years.
Now if the price of new condition homes in the area rises from $100/sf to $175/sf, a lot now offers someone the ability to build an ugly 4,000sf McMansion amongst the stately old homes for $150/sf and sell it to a douchebag for $175/sf. The person ordering the construction of said monstrosity might be willing to pay tens of thousands of dollars for the option to build it. E.g. They might pay you $70k for the lot so they can make $30-50k just for tying up their capital a few months.
Depending on how far prices must rise to exceed the cost of construction, lots might be a high-risk high-reward investment. However, if you win this gamble, you will regret not moving into the neighborhood yourself. If prices went from $100/sf to $175/sf, you might have made much more money owning a completed home there, all while skipping the extra lawn mowing and trash pickup.
Also, it sucks to pay $1k in closing costs to buy a $12k lot that will require another $1k to sell.