Lawyers have mostly looked at the common open source licenses long enough to know what they say: what is compatible with each other, what is compatible with their [corporate/clients] other needs, and otherwise how it works. While mostly they have not been tested in court, there general consensus they will hold up somehow (nobody knows for sure and there are a couple hundred countries that could each decide their own thing).

When you do your own license though lawyers need to figure everything out for just your project.