I am not so sure we really have or had both. Maybe for roles like electrician and tiler and such. But for roles like teachers, I feel it is not the case, or policy makers are extremely stupid here. The hurdles for someone to become a teacher without having studied that are still very high. It doesn't seem like policies a nation would implement, if they really wanted more teachers. For example you need to have 500h of base level pedagogy training (which would be OK) and of course you need to be very knowledgeable in the subject you want to teach (also makes sense), but that's not enough, no, they want, that usually after 3 years you teach 2 subjects, not only the one you are knowledgeable in. So you are expected to either have sufficient knowledge (lectures at university count towards that, but may well not be sufficient) in a second subject, or to somehow catch up on that.

In my case, I did consider becoming a teacher for IT stuff. Not really "IT", as pupils would first learn the basics of course, and the really interesting stuff might not even fit into the curriculum, but I could well imagine teaching what I know and maybe even establishing an extracurricular computer programming club or something. I would feel great passing on my knowledge. But what second subject to teach? ... The only other applicable thing I had in some amount during my studies is ... math. Ugh. You telling me I might have to take additional math lectures, with people who study to become math teachers?! No friggin way man. Also am not a native English speaker, and while I use English every day all day long, I don't think I have the detailed knowledge to teach it responsibly to students, unless they are total beginners, but I won't be teaching IT stuff in primary school, now will I.

If you stick to only one subject, perhaps they will tolerate you, but your salary will forever be a reduced one, even if you work the same hours, just because you only do one subject, and no matter how much of an expert you are.

When it comes to IT/software engineering I don't think there ever was a shortage. Just businesses, implementing big tech hiring processes for their tiny startups and then playing surprised pikachu, that they are "not finding talent".