No one is telling students to use or not use parallelism, it depends on the workload. If your workload is highly sequential, multi-threading won’t help you much, no matter how many cores you have. So you might be able to switch out the algorithm and go with a different one that accomplishes the same job. Or you re-order tasks and rethink how you’re using the data you have available.
Practical example: The game Factorio. It has thousands of conveyor belts that have to move items in a deterministic way. As to not mess things up this part of the game ran on a single thread to calculate where everything landed (as belts can intersect, items can block each other and so on). With some clever tricks they rebuilt how it works, which allowed them to safely spread the workload over several cores (at least for groups of belts). Bit of a write-up here (under “Multithreaded belts”).
Teaching software development involves teaching the theory. Without that you would have a difficult time to decide what can and what can’t benefit from multi-threading. Absolutely no one says “never multi-thread!” or “always multi-thread!”, if you had a teacher like that then they sucked.
Learning about Amdahl’s law was a tiny part of my university course. A much bigger part was actually multi-threading programs, working around deadlocks, doing performance testing and so on. You’re acting as if the teacher shows you Amdahl’s law and then says “Obviously this means multi-threading isn’t worth it, let’s move on to the next topic”.