I had the good fortune to start my online genealogical searching on people who lived in Chicago, and then moved to California. Both Ancestry.com and Family Search web sites provide a number of collections of BMD records from the late 19th to the mid-20th centuries, making it possible to find information on people's children, spouses and parents.
Recently, I've had to do some searching for similar records for people who lived in New York City, and I have been surprised by the lack of such collections. Yes, I can find marriages (1894-1937) through stevemorse.org but births are only available in the (1880-1909) range, and parents' names are not indexed.
For deaths, the Illinois death index (1916-1947) may list parents and spouses names, occupations, the cemetery where the person was buried, etc. Nothing similar seems to exist for New York.
The odd thing is that collections do exist, but there aren't very many records in them. The New York deaths and burials 1795-1952 collection, for example, only has 682,387 records in it.
What are the reasons for these differences?