OK, that's a slight exaggeration. And most of them are concentrated between Summit and Union Station. I also know of CREATE's long term plan to bridge over the Brighton Park interlocking. But for the near term, why do Metra and Amtrak trains have to crawl along this whole stretch at 30 mph? All these crossings are at least 1.2 miles apart. There's no clear reason why trains can't be permitted to go 40 or 50 mph between interlockings, especially on a favorable signal indication. I can see shaving 90 to 120 seconds off the travel time here. More than that, it would end the perception of continuous, nonsensical creeping that now takes place for the first 8 miles out of Chicago.
Well, the short answer is that CN owns the railroad, and as such, they get to impose whatever speed limits they see fit.
But the real answer lies in a combination of several factors, which include curvature, turnouts, bridges, and most importantly, track condition. Metra/Amtrak trains have to commonly negotiate through several 10mph crossovers on the Freeport sub (between 21st St. Jct. and Bridgeport, used by passenger traffic to connect Amtrak's terminal district with CN's Joliet sub) and the overall speed is further declined by the several curves along this stretch as well. Last time I rode this route (2 years ago I think), there were also several sections of jointed rail on the Freeport sub segment which is another contributing factor; although this may have been replaced since.
And speaking of Brighton Park: Until several years ago, this was a non-interlocked crossing with semaphore signals controlled by an on site switchtender. All traffic through here was required to stop first, then proceed on signal indication. (An antiquated operation true, but it certainly attracted its fair share of railfans from around the world for this very reason). Today, this crossing is now controlled remotely from NS's Ashland Ave. yard, which has sped things up a little, but real improvement won't occur until the flyover is built. For now, we'll just have to take what we can get...