There's the issue to me. Have all the pizza parties you want, but I hate the undertones that you're not a team player if you don't show up to a voluntary function after work hours.
What kind of party is it if you aren't paying me to be there?
When I was younger though (and child free), I'd jump at these sort of events, and loved them. A company I used to work for wrapped up about 6pm, except Friday when you were still expected to be around until 6, but there were beers and soft drinks plus snacks from 5pm. Most people would still end up talking work, and plenty of us would still be there talking work (in various guises) until 7pm or later, and would carry on to the pub to do the same if we fancied it. But I loved the fact it started in 'working hours' and that the snr staff would all get involved. Was a great icebreaker and helped move silos between teams and departments.
Now I'm older, if it is outside of working hours, I have to see the benefit in it for me.
I think this is the key point. After hours parties only focus on one demographic (young and/or without children). Some non-zero percentage of the workforce will not fall into this category. People organizing such parties should not be surprised that such people don't show up.
Now, some companies do have after-hours holiday parties, but those generally feature a higher level of food than pizza -- also booze.
Those aren't nearly as common as they were years ago, though (too much liability from the booze factor, I reckon).