On one hand, I can see why a provider would like to know. I do think if someome takes a large enough dose it will be exhibited well and can be simply observed, but it might be too late to prepare by then.
On the other, I can think of multiple reasons why clients wouldn't want to tell about this :
1. Simple embarrassment. Some people especially younger ones are often embarrassed they have to take these drugs and can't get good hard ons even with sufficient visual/physical stimulation. If they are embarrassed to admit it to a doctor they might be very embarrassed to admit it to a good looking, good service woman.
2. Precisely because the provider might overprepare to the point of taking off menu items she otherwise would have. If your included msog is now off the table because I popped a pill, that doesn't feel great in terms of value.
Tangent story - I was seeing three girls on one day and the last one I lasted a quite long time with. She was frustrated at me not cumming fast so she asked if I have jacked off. To that I replied with the truth - I have seen someone earlier in the day. Didn't even say it was another escort. Could've been a SO. But she got upset, denied me msog and told me to never see anyone on the day I see her. Well OK, I just stopped telling anyone I have seen other ppl. I'd like to be honest but if honesty gets me into such situations they don't need to know about it then. I don't see restaurants getting upset if you dined elsewhere before. Anyway.
Back to the topic, I already mentioned how it doesn't feel good if menu items are taken off. If provider starts to limit when she can see you because she doesnt want a long ground and pound session, this can be detrimental to your schedule too. Not sure how to alleviate this. Many clients already feel bad about taking pills to stay hard, being penalized for it sucks imo. But I also get the provider side. Guys with diamond cutters for dicks who wanna fuck for the whole hour probably aren't their favorite clients.
3. Being afraid of some pushback. This goes along points 1 and 2. If I tell a provider I haven't seen before I'm gonna take pills and I'm not too old, I don't know what to expect. Some clients might be scared to be rejected or treated differently than they otherwise would.