trouble with making zombies spawn at pigs

I'm trying to make a zombie with modified attributes spawn at every pig with a 5% chance. I can tell it works bc when I use a spawn egg I DO get some zombies, but I flew around the overworld for 15 mins and saw hundreds of pigs but no zombies (they can survive in daylight). what I think is happening is that they're despawning, so how can I avoid this, besides adding PersistenceRequired? because I think that could get laggy since I have commands running at each one