That's not true. In Halo 4 you can, in fact, join games in progress if there are slots open. Either way that doesn't relate to the post at all. I am also having the same issue. Seems to be either something with our crappy ISP's or something with the 343
Halo 4 servers. I see about 1000 people online total, whereas my friend has 10x in Infinity Slayer. Not sure what exactly the issue is, but no you're not the only one. Hopefully there is a fix soon.