Well, most firms have two graduate intakes each year, so that's some new trainee staff every 6 months, who each take longer than 6 months to "train" - some training can take 1-5 years in professional roles.
Continuity and Consistency when training raw recruits is essential - you can't have 5 different people training up a brand new raw recruit - a different person on each different day of the week. It just doesn't work.
Firms are reporting that new trainees are taking longer to get up to speed, taking longer to learn "soft" skills, etc. It's not surprising if the more senior staff training them are located somewhere else or they have a succession of different people trying to train them.
Even "experienced" new staff, i.e. who've been recruited from other firms, still need training in the new employer's systems, ethos, structure, etc. I've moved from one accountancy practice to another a few times, and each time, despite me being experienced and qualified, it's been several weeks before I was fully up to speed with different software, different filing/admin systems, different staff/management structures which has necessitated lots of "questions" directly at the staff on nearby desks over many weeks. When you're set next to others, you can choose the right moment to ask questions, i.e. you can avoid when they're stressed, clearly busy, etc or ask someone else - try doing that remotely and you're just going to annoy the person you constantly interrupt and start being reluctant to contact them.