The dirt simple power supply like that actually isn’t too bad with respect to efficiency. They are usually up around 90% or so. The problem is that there is no voltage regulation. AC power can vary. It used to be that the power company guaranteed +/- 10%. Most of them these days guarantee +/- 5%. Your power company’s web site probably has what they guarantee somewhere on it, if you are curious.
Since it is unregulated, the AC will vary between say 109 and 121 volts (assuming they are aiming for 115 volts, +/- 5%), which means your 24 volt output will likewise vary from about 22.8 to 25.2 volts. This could make a visible difference in the brightness of your lamp.
The power supply would also be physically larger and heavier, due mostly to the transformer. For smaller power supplies, it would also be cheaper, but for something in the range of 150 to 200 watts or so the transformer is going to be fairly large, which means a lot of expensive metal.
If you add a simple linear voltage regulator, those don’t do so well from an efficiency point of view. Your overall efficiency is likely to drop to somewhere around 50% or so,with the linear regulator producing a lot of waste heat as a result. But then your 24 volts would always be close to 24 volts. Since a linear regulator basically bolts onto the back end of your simple power supply, it adds to both size and complexity, increasing the cost accordingly.
A typical switch mode power supply eliminates the huge bulky transformer and uses much smaller transformers (operating at a much higher frequency, which is how they can be so much smaller and lighter). The circuitry is much more complex, so for smaller power supplies the cost of all of the extra circuitry outweighs the cost of the simple transformer. This is why your typical wall warts are basically the simple power supply with a linear regulator (some of them even just have a rectifier and don’t have the regulator). Once you get above a certain power level, then the extra cost of the switch mode circuitry begins to outweigh the cost of the huge metal transformer, and you also get a huge benefit with respect to waste heat, which also becomes much more of an issue as the wattage goes up.