Primarily because it is not reasonable to ask recording schemes (lots of which are run by just one person or by voluntary labour, frequently both) to trawl the vast array of online sites where records might be posted and harvest them themselves. Similarly when I post to the various email discussion groups and other online forums I don't expect the recording schemes to be carefully capturing all the records from them.
iSpot of course is remarkably poorly designed to function as a recording scheme (no field for number of records, not field for stage, no way to bulk upload data etc.) so clearly is never going to be able to take over as the central database of any scheme. That's not a criticism of iSpot, back when I got involved (2008/09 I think) it was made quite clear that its purpose was not to be a recording scheme, so it not being designed to be one is very reasonable.