Don't overthink why Colin Kaepernick is still a free agent. Simply put, years have passed since he was an effective quarterback. He is 29 years old, has succeeded only in an unsustainable scheme and is part of a well-populated group of former starters who also remained available as the week began.

Filmmaker Spike Lee has raised the issue of whether social and cultural forces are at work here, calling it "fishy" that Kaepernick is unsigned. The implication, of course, is that teams are avoiding Kaepernick because of his decision to kneel last year during the national anthem.

To blame Kaepernick's unemployment on his protest is to misread the way NFL teams make personnel decisions.

While it would be naïve to think every owner and general manager approved of Kaepernick's protest, or that it never influenced their perception of him, it's important to understand priorities. Teams are swayed first by a player's potential (or lack thereof) to help them win. Protest or not, Kaepernick would be under contract now if he had played demonstrably better in recent years.

Let's look closer at where Kaepernick is as a quarterback and why that has left him sitting in a group that also includes fellow free agents Jay Cutler, Josh McCown, Ryan Fitzpatrick and Robert Griffin III.

Kaepernick's greatest NFL successes came as part of the San Francisco 49ers' read-option scheme from 2012-14. In those three years, he rushed for 1,578 yards -- more than any quarterback other than Russell Wilson and Cam Newton. His success -- he threw more than twice as many touchdown passes (50) as interceptions (21) -- helped him compile the eighth-best Total Quarterback Rating in the NFL (70.2) over that period.

Even then, Kaepernick was one of the league's least-accurate quarterbacks. His 60.1 completion percentage ranked No. 23 in the NFL, and his percentage of off-target throws -- judged on video by ESPN Stats & Information -- ranked No. 18 (17.6 percent).

Those issues intensified in 2015 and 2016 amid the 49ers' coaching turmoil and talent drain. Since the start of the 2015 season, Kaepernick ranks last in the NFL among 35 qualified passers in off-target percentage (22.6). His completion percentage ranks No. 32 at 59.1. He still was one of the NFL's most productive rushing quarterbacks, ranking No. 4 in total yardage over those years, but it didn't mitigate his passing deterioration.

There is no more important attribute for a quarterback than accuracy, especially for a free agent who is shopping himself to teams with various schemes. In the long term, a quarterback's running ability is considered a complementary skill and one that historically fades with age or because of injuries. Scrambling ability and arm strength are secondary to whether you can hit the target.

All passers miss throws, but Kaepernick over the past two seasons has done it as much or as more as anyone. It's hardly surprising that teams didn't rush to sign him in the opening days of free agency. Realistically, he's vying for the kind of veteran backup job that often doesn't get filled until after the draft or later. That deal is going to be worth a fraction of the $14 million he earned last season.

The concerns of Lee and others are not completely unfounded. There is precedent for NFL teams passing over players who engage in social activism. The sudden end to punter Chris Kluwe's career in 2013, a year after he waded into the issue of marriage equality, comes to mind.

But fair or not, those decisions are made on a sliding scale. The better player you are, the less teams will heed their off-field concerns. Kluwe hadn't kicked well, compared to the rest of the league, in 2012. Kaepernick has been one of the league's least accurate quarterbacks for two years.

When given a choice between players with relatively equal projections in terms of production, teams are likely to choose the one who brings what they perceive to be less controversy. What happens on the field is always the most important factor. So it goes.