Usually because we can't afford them and they are too much capability for us. It's kinda funny that we're calling that our new ships are "destroyers" with only 24 VLS cells. Compare that to what an American Arleigh Burke has. And then look at how much ammo some destroyers expended during engagements with the Houthis.
Cost for US Warships is an issue, but the way they design them for crewing is more so. They have a lot more people on their ships.
The number of VLS cells does not determine what a ship is called. Truthfully, it's becoming more of a statement then anything else. Frigate, destroyer, cruiser, and battleship used to have specific meanings and roles, but that has gone by the wayside.
We called the St Laurents et al Destroyers (DDH), but they were only 2260 tons, and in reality had no air defence. The Iroquois class were also destroyers at 4400 tons, but the Halifax was called a frigate at 4735. The Rivers are 8000, which is comparable to a Burke Flight I at 8300.
The Rivers are a very capable all round surface combattant, but are not optimized for area air defence as much as the Burke. The are also intended to carry AEGIS as the combat system.
The latest flights of the Burkes are not really destroyers. They've taken on the capability and role of the retiring Ticos, and are also as large (a Tico is 9600 tons). In other words, they are specialized for area air defence and for all intents and purposes are cruisers (in the old way of speaking).