I figured I’d try and see if there’s any reasoning given, but all I can find from my limited google-fu is hearsay. The only people who know why Funcom didn’t give multiple character slots by default (barring the 2 free ones for TSW linked accounts at the first Legacy Transfer) are Funcom themselves. Presumably, at the time it was thought to be an economically viable route with the information they had, and to change that now would either cause a controversy or else remain non-viable, or they would have presumably done it by now.
If you haven’t done so already, add it to https://forums.funcom.com/t/feedback-suggestions/
Reading through FAQ - Secret World Legends no reasoning is given as to why, but merely a statement of fact:
How many characters do I get on my account?
Each account has access to one character slot. Additional character slots may be purchased in game.
Players that used the original TSW Legacy Transfer service on or before September 4th, 2017 will have three total character slots. The Legacy Transfer service no longer grants additional character slots. Read here for details.
Though I did find one of the web results from trying to search for a reason includes Meta-Moth’s post from May 2018 Extra Char Slots NOT available - #17 by Starsmith, in which he gives the excellent summary:
I have no verds
https://zippy.gfycat.com/WearyImmaculateGelada.webm
As one person states underneath that:
From the seller’s - i.e. Funcom’s - perspective, which unlike [Meta-Moth’s] is actually relevant to the question of whether said seller would be losing anything, any Aurum item really is a cash only item [in this case referring to free character slots].
They may not care whether the person spending the cash and the person getting the item are one and the same, but the item is not going anywhere without somebody having spent cash. And therefore they’d be (potentially) losing cash by giving it away that item for free.