First off, the body of material covered by the professional engineering exam is based entirely around other disciplines, so I will assume we're talking about a hypothetical new exam. Let's start with a basic logistical question - what would you even test on?
If you want to evaluate coding practices, you have to pick a language - and there is no single right answer. Many of the common languages are even tied in some way to a major tech company, so supporting some but not all could be seen as biased.
Ok, so maybe coding is out, but engineers should at least know the process for gathering requirements and turning that into working software, right? Great, so are we testing on Waterfall, Agile, Kanban or something else? Best part is, each of those will teach you that the other methodologies produce terrible results. Besides, you can already do certifications from various vendors to show you have those skills.
Except, here's what's interesting - the companies that seem to be the best at software engineering care the least whether you have those certifications - probably because they've found that virtually anyone can memorize some content to fill out a test, so it's more meaningful to put them on the spot and ask them questions they aren't expecting.