Well it's because that's the time when CPUs were any useful in mining coins.
You can try mining a coin using the CPU on those computers but just for fun. Don't expect to get coins that will have any value.
If you're too lucky, you'll get a dollar per month which would of course be useless since you'll pay much more on electricity to run them.
EDIT: I've found a coin that is relatively new and the devs impose that it's cpu mineable only.
https://bitcointalksearch.org/topic/ann-cpu-mining-yenten-v201-ytn-yescryptr16-exchange-avilable-2329470
I'm not sure if it's legit and I hold no responsibility for you mining this coin. You said that those CPUs are low-grade ? I'm not sure how it'll turn out with you. But I guess you won't get anything.
OR try investing in some mining hardware (build a gpu mining rig ?) if you're serious about this idea.