Sales funnel + psychology. Imagine you are using PHP4 and, 100 candidates out of your pool will totally hate it. As You know that in a face-to-face talk you have about a 10% chance to convince a PHP4 hater to still go ahead and take the offer. Now consider 2 strategies:

A) Write "we use PHP4" in the job description. In this case, 98 candidates will nope out when they read this (say 2% won't notice PHP4 or will be desperate) and 10% out of the remaining 2 gives you 0.2 people on average.

B) Be as vague as possible. Write that you use modern tooling and technologies, don't give any details. Now 80 candidates proceed to the next stage. If you only convince 10% of them, you still end up with 8 instead of 0.2.

It's similar to spam - even though >99% people will delete it without even reading, as long as there is a tiniest chance of someone actually buying the advertised product, the spammers will be in the black.