The calculus and probability skills are used heavily in those later exams. I think that Exam 2 can be delayed until whenever the student has learned the material, possibly well after Exam 3. You don't need a lot of the Exam 2 material for Exams 3 and 4. But Exams 3 and 4 are mathematical, and it might be wise to take them fairly soon after your mathematical coursework. Students who took calculus and probability recently should be able to prepare for Exam 1 without a seminar.
Textbooks are well written and there are solid study manuals available as well.
I imagine that the growing availability of Exam 1 seminars is primarily for people who took the classes a few years earlier. The University of Illinois offers coursework that covers all the material on the first four exams and we do review seminars for our students on Exams 1 and 2. Seminars for those first two exams are certainly not absolutely necessary for candidates, but they can be helpful, especially for working through lots of exam-type problems. The textbooks for Exams 3 and 4 are of very uneven quality, uneven difficulty and uneven mathematical sophistication. Many students self-studying from those books have difficulty judging what is important and what isn't, which probably explains the larger number of exam prep seminars for those exams.