From what I've seen in other MMOs such as SWTOR and ESO, the short text and the like should be done in say, etc, this gets people interested. But the large text prompts should not be used through say because with the size of the text box and all that, it pushes the entire chat upwards. This is why people hate it when folks constantly post FC advertisements. It's not a lack of common courtesy, on my behalf - it's something where large text prompts for RP should not be used through say. From what I gathered from the usage of the RP tag is that you can see certain people who are interested in RP and can message them directly. That's honestly what I thought it was for.
It's not next-level thinking. Thankfully I am not on an RP server, however in SWTOR and ESO the first thing I do is make specific chats that filter this out because people will write large paragraphs and throw it into say. I personally find that inconsiderate. Just like you think me not liking it is lacking common courtesy and or empathy. Unless I'm reading what you're saying incorrectly if so, I apologise as text sadly doesn't have tone.
Of course, you're going to get people who don't do this and thus start acting like gits when they see RP pushing their chat upwards, or they're just there to ruin your day. Either way, I don't see why it matters why me wondering why folks don't use party/LS to do this - I honestly think it's much better than using say because then it's not broadcasted publically and people can be added into the party/LS to enjoy RPing without having people looking in. Shrug.
Finally, as it has occurred to me that you are using sarcasm, tsk. You can't use the defence of a social game when the chat is completely dominated by people using large text prompts - I'm talking about multiple lines of text one after with the use of say that pushes all of the other social aspects of the game upwards, such as FC, Party, Tell, etc. Thankfully say is proximity-based, least they're not doing it in Yell/Shout.