Salaryshare and the data opportunity for jobs aggregators

It always worries me when I see companies launch that it seems any one of a dozen other companies could trivially kill with a few days dev work. I wonder if I'm missing something.

SalaryShare lets you see what your friends/colleagues/random strangers earn by pooling salary data anonymously from anyone who chooses to share it and then revealing the range of salaries disclosed to all participants. Nice idea. I have indeed occasionally been curious as to what other people earn.

Thing is...if people want to know that badly, can't one of the jobs boards (or more usefully jobs aggregators) simply store data on the salaries that were listed on historical job ads and make it searchable?

If I want to know what my CFO earns a simple search option on eg Jobsite or Indeed could let me trawl the database of old job ads, stick in some parameters like job title, company and approximately when I thought she joined, and hey presto I can see the old ad she applied for and therefore the salary range. Maybe she haggled that up a bit but it'd do as a ballpark and be more accurate than the range from a pool thing from Salaryshare. And I don't even have to get her to agree.

What am I missing here? Perhaps the jobs aggregators don't (or for some reason can't) store that data. but otherwise - if there really is demand for this sort of thing - it seems like an obvious space for them to get into. Nice hook to get people in to their sites, nice bit of PR, off the top of my head it'd be fairly easy to implement off their existing search tech. In which case Salaryshare would have a bit of a problem but them's the breaks.