I hear this argument a lot and I agree 100% that this is the reason, but I'm baffled about why developers feel like it's so hard to review someone's GitHub profile. I find them shockingly easy to evaluate, as in literally it takes about a minute. Here's what the process looks like:
1. Look at the project list, if it's all tutorial projects, you're already done. There's no use looking closer because you won't be able to tell if it's their source code or if it's copied from a tutorial.
2. If it looks like they have some real source code, apps or frameworks they've written from scratch, pick one and look for an important file. There's no trick to identifying an important file, in most cases the projects are small enough that it's obvious. (And if the candidate has large and complex projects, you've probably already identified an exceptional candidate.)
3. Once you've opened the file, I'm always stunned at how easy it is to evaluate. I'd say in about ten seconds of scanning I've usually identified three or so mistakes that roughly gauge that developers ability. In which case I'm done, unless I want to dig for some material to discuss on a phone interview.
4. If I haven't seen three mistakes, then we have an exceptional candidate, this is really rare (maybe 1 in 100), so we've already identified an exceptional candidate in about a minute.
One of the keys takeaways is that process only takes a long time for a truly exceptional candidate, in which case I'm happy to spend the extra time. I'm baffled by the preference for coding tests, because those I find way harder to evaluate. I know in my bones what production ready source code looks like, because I've written and read it all day, everyday for ten some odd years. Evaluating someones coding test is way more mental gymnastics and guesswork for me.
For the people who have experience evaluating GitHub profiles, but instead prefer giving tests, I'd love to hear how your experience deviates from mine. I know there's nothing special about my ability to evaluate source code, e.g., I watch my colleagues do it all day as part of code review.