As someone who fought hard to switch careers into this industry and teaches software engineering now I understand the impulse to think this but I have to say all my experience thus far contradicts it.

Which part(s)?

Possibly replying to someone else.

Why does web dev not use much formal cs in your opinion?

This is an interesting question. From personal experience, I have also found simple GUI work to the "digging ditches" work of programmers. (Weirdly, I still love it, because it makes it easy for end-users to understand your tools!) Of course, complex GUIs are another beast, but usually GUI programming on the low end as it is very hard to test (easily) and requires endless tweaks due to the high configurability of GUIs.

For example, if you just look at GUIs created using Excel/VBA, you will find they seem to need endless tweaking to work just right (edge cases?), but do not at all require a formal CS degree. (I have worked with many non-CS people who are fine a mid-level Excel/VBA stuff.)