Example 1: they need to get a rough idea of whether you're a jerk, whether you can communicate in the required language, whether your resume is BS, etc. Remember, nobody's paying the recruiter to find you a job - somebody's paying them to find high-quality candidates. Their job is to sell you, so naturally they want to know what they're selling.

Example 2: simple context switching. If your job required you to be involved in dozens or hundreds of simultaneous ongoing negotiations, wouldn't you find it more efficient to block out 30-minute calls with each stakeholder, rather than jumping around between a hundred different email threads full of single-question emails?

One could probably come up with more straightforward reasons. As a general rule though, nobody is ever consciously trying to waste your time. They're trying not to waste their own.