Calendly is useful for scheduling meetings with people outside of your company. For meetings inside of your company Outlook and Google Calendar already have built in scheduling tools to help find a time that works for everyone.
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.
