I grab tons of free games, hardly play any of them.
True, but look at the numbers. It increased to a "current load" of 4,177 with a peak of 33,467 players.
About 65 million users use steam daily, and there are about 125 million monthly active users, so even while this is a sevenfold increase in Half Life players, we are still only seeing 0.02% of Steam users playing Half Life at its recent peak.
It's totally possible that - like - a million people downloaded it, of which ~3.3% decided to play it.
Most people probably only download it, and then never get around to playing it, but a small percentage will play it, and for a game that was released 25 years ago, and that otherwise would have very few players, even that very small percentage of a decent number of people can amount to a significant bump in the stats.
Edit:
The hours played chart is pretty cool:
It was averaging about 10k hours of total playtime across all of Steam per day up until it went free, and then it peaked at 586,738 hours on November 18th, but has since dropped down to about 147k hours as of yesterday.
The truth is, while I struggled with Half Life back in 1998 when it came out, and it took me many hours to play through it, that was on my aforementioned Pentium 150 (pre-MMX) at 200Mhz, with my 6MB Voodoo 1 at 640x480 and 16bit color.
The framerate was not awesome. This was before the era of "60fps is considered playable". I have no recollection of the framerate I was actually getting at the time, but retro gaming forums suggest Half Life got 15-25fps depending on the scene on a Voodoo1. I probably had vsync on as well, making the input lag worse.
These days you could totally play through Half Life in a single sitting if you were determined enough. Higher framerates make the jumping challenges and fights so much easier even on the most difficult settings, and if you get stuck in a puzzle, most people will just google it after a few minutes of not being able to figure it out.
So, even if those who downloaded it and actually play it stick with it all the way through, it's probably only going to be a blip in the stats. They'll be at the end credits within like 8 hours.
)