People are the capacity. Who is on which project, at what percentage and until when is the entire planning problem, and a studio that promises a start date without checking it has already broken something else.
Milestones are contractual events. A build delivered, reviewed and accepted releases a payment, and the record of what was submitted and when is what prevents an argument about a late invoice.
Scope moves constantly and it has to be priced. A change request written, estimated and accepted before work begins is normal practice; the same change absorbed to keep a client happy is how a profitable project stops being one.
Recorded time makes the next quote possible. Knowing what a comparable feature actually took, rather than what was estimated, is the only way an estimate improves rather than repeating the same optimism.