Usually because of licensing or credential assessment rather than the immigration step. Both depend on third parties with published but inflexible processing times.
The delay is upstream
When a start date moves, the cause is rarely the port presentation or the petition. It may be a regulator, examination schedule or credential assessment waiting on a transcript. Those steps are upstream of everything else and cannot be compressed merely by attention or urgency.
The practical response is to ask, at the very beginning, what each third party's current published processing time and schedule is, and to start the longest one first. It is also worth asking the employer what happens commercially if the date moves, because that conversation is easier before a plan exists than after it has been announced. A period requested in the employer letter should reflect the real engagement rather than a maximum, since an extension is a fresh assessment against the same requirements rather than an automatic renewal.