Superstition/caution.

They aren't 'difficult' to cook. They are dangerous to eat if uncooked (and thus undercooked).

While true morels themselves can be dangerous while uncooked, there are similar looking species that are both less and more dangerous.

Species of Gyromitra or "false morels" like Verpa Bohemica will commonly all be called "morels": both as an intentional cultural colloquialism or simple misidentification.

Depending on which hemisphere you live in, some Gyromitra species may be more dangerous than true morels. They can also be more dense and harder to cook thoroughly.

Most mushroom species will cause an upset stomach if undercooked. Drying is an effective way of reducing both dangerous and uncomfortable compounds. Note that not all compounds are destroyed! "Magic mushrooms" are famously traded dry for example.

The advise to add an additional preparation step increases the chance someone will notice the wrong species hiding in their ingredients. Undesirable species can also have overlapping habitats and climates so its not uncommon for a careless or ignorant forager to pick the wrong thing.

Morels contain several volatile compounds which cause gastric distress. (Forgive me for not looking it up at the moment, but one of them is/was a compenent of rocket fuel, which teenage me loved.) They have to be thoroughly cooked to burn those off. Or else dried.

Specifically for soup - which is, arguably, their best use - most people won't saute morels long enough before adding liquid, so it's always best to use dried for that. Otherwise, standard, boring, dry-sautéed + butter until tender works great, and has never given me a hint of upset.

The instructor of your friend's mushroom course may have been giving maximally-cautious advice, rather than trying to communicate nuance to the general public. That's often a wise choice. :-)

PS. If you're at all interested in foraging mushrooms, buy a copy of All the Rain Promises and More, by David Aurora. (If you're elsewhere than North America, buy a local guide, too, but still get ARPM.) Aside from the mushroom content it's wonderfully entertaining.

I've never heard of this. In fact, I'd go so far to say myself and those that taught me try our best to not let them dry out. Can you explain further?