Have you looked at the prices of high-power resistors and rheostats (variable high-power resistors)? :) That's just a bit tongue in cheek, but those are not necessarily cheap - especially not if you want to mount them on a heatsink.
Electronic loads don't need to be either complex or expensive. A basic one, usable for up to 100W or so can be cobbled together from parts found inside a junker PC, quite literally. CPU heatsink + fan to dissipate the heat, mosfets from the ATX power supply and/or onboard supplies as series pass elements, and a couple cheap op-amps. Of course there's also the knowledge of how to put it all together. But if you had a bunch of junk desktop PCs, you could probably make one from nothing but parts scavenged from those PCs, quite literally.
Of course that would be a relatively nice adjustable load. If you only need a "dumb resistor" then of course you can use resistors - they just get expensive to implement, it's somewhat cheaper to use junk parts if you got them.
There are YouTube videos where you can see an electronic load being built start-to-finish. Some of those present quite reasonably performing designs built for very cheap.
If your time is free, then you can get quite a bit of lab gear made the DIY way, and it will be much "cheaper" in material cost than off-the-shelf devices. For most people professionally into electronics, though, their time is anything but free, and they may have different hobby projects in mind than what they need on the bench at work.