Where could you find a metric for global cash payments per second? You could ask a store to tell you their payments per second, but there's no central point where you can monitor and collect that metric.
Where could you find a metric for internet packets being transmitted per second? I mean you can ask a server, and heck you could probably get a good estimate from Cloudflare and Amazon since they are middle men for most internet traffic, but it isn't everything.
Same for lightning. You could probably ask WoS since they probably do a lot of transactions, but you can't ever know what you don't know.