You're ignoring two things: 1. People's selfishness and 2. People of older age > Any group who actually thinks about it would quickly realise that 1 person off

You're ignoring two things: 1. People's selfishness and 2. People of older age > Any group who actually thinks about it would quickly realise that 1 person off for 2 weeks with the flu is better than 10 people off for 2 days each with the flu. People tend to not think that way. It'll not be "luckily David stayed home so we don't get sick" but "David stayed home again and we didn't meet the requirements, screw you David!" Older people ("older" as in 45+) are more often sick due to age related problems like back pain, knee problems, etc. Having those people on your team means that you are disadvantaged in comparison to other teams that don't have so many older people. People notice those things and will try to get rid of the older people to get their cut. To the company the experience of older people matters but their colleagues only see their bonuses disappear. This creates hostility between age groups and (imho) leads to ageism.