The reason, btw, is because even though the senders *send to* a stealth address, they can log the original silent payment address (or monero address, or bolt12 offer, or in general any reusable payment string) and (1) use that for profiling -- every time someone asks them to send money to it, it's another data point in their profile on that payment string (2) share data with other senders about times when they sent money to that payment string, that way, together, the colluding senders know more than they would know individually