It's a tax issue. Illinois has lots of those, unfortunately.
Elburn is in Kane County, who belongs to the RTA's tax district. Metra could extend another 8 miles to downtown Maple Park, which is half in Kane County, and still be "legal"....
To extend the full 16 miles into DeKalb would require the entirety of DeKalb County voting to join the RTA and agree to what is currently a 0.75% sales tax, of which half would be spent in-county. Sandwich, literally just inside the DeKalb county line from Kane County, can't have service, either without that DeKalb County vote.
Just like extending Elburn to Dekalb would be a no-brainer under non-BizarroWorld situations, extending Metra to serve Belvedere or Rockford despite only being 12 and 30 miles west of Marengo (the furthest west station in McHenry County) is out of the question, as that would require Boone and Winnebago Counties to agree to the 0.75% tax.
I've said for a long time that the sales tax requirement is the biggest roadblock to having reasonable extensions of Metra as the suburbs grow into what used to be exurbs and farmland.
Short of modifying the RTA's underlying state law, it's apparently easier to throw millions upon millions to do studies to subsidize Amtrak.
Bus service for the last mile seems to work for the couple thousand people that actually use it... and it doesn't cost millions upon millions, and doesn't require taxing people for little to no benefit.