It's all the potential criteria around the request that makes just that one piece more complex than it seems.
For example; "Make me a site that does x, you have 2 hours" Do I need to build the app in front of you? Problematic because how often does one build an entire web site in a couple of hours, while someone is standing over their shoulders scrutinizing every keystroke?
Do I take it home and do it? If so, how do I prove to you that I did the work?
Do you care what framework I use? Do you care if I use Open Source code?
What design should I use? Where do I get the site assets from?
If I'm building this site for you, are you paying me for that time?
Then there's the non-technical parts missing from an interview like that:
How does such an exercise tell you that I'm the kind of person you want to work with?
How does such an interview provide me an opportunity to ask you question? It tells me nothing about you or your company which is every bit as important as you finding out about me.
if your an app dev: make me an app that does x, you have 60 min
if your a rails dev: make me a site that does y, you have 2hrs
if your a pdm: make me a spec and tickets for a system that does z
maybe i'm missing something but it shouldn't be too hard should it?