Even then Outlook does have FindTime and has a plugin system to let you use third party tools like Calendly. There's also a product called Microsoft Bookings that's more of a direct competitor to Calendly. But at the end of the day Calendly's revenue is estimated at around $100 million from what I could find online and that's a drop in the bucket for Microsoft or Google.
Edit: someone mentioned in thread that Gsuite supports this natively now, so I suppose my question is more MS focused.