Because it will benefit the RMT greatly.
In my opinion a jump potion would likely be a good thing for the game, but only if it is applied correctly. I don't believe charging extra for a new player to reach more recent content is the way to go about it though. My suggestion would be for any new player, or player below level 50, who has purchased Stormblood, to be able to advance that character to level 50/60 on their main class only, and only once. Perhaps give the option for people to get a fresh 50/60 on an Alt only once as well, for those who may have interest in it. It will probably have a short term harm on the game but it is likely necessary for the game to survive in the long term.
I don't believe there is an argument to make job potions available for every class other than your chosen starting class, as it would make the game pay-to-win somewhat (sink enough money in and get all 50/60s is a bit silly). If job potions don't happen, perhaps a more streamlined and shortened MSQline would be a better solution anyway.
I've yet to see any RMT bots spamming their nonsense in Ishgard's marketboard and aetheryte areas. This is primarily due to having to delete their characters after a short amount of time and create a new one, and new characters have to go through the entirety of ARR's MSQ to set foot in Ishgard. Yes, RMT is clearly willing to pay $ for the sub, but that's a one time fee for a large number of character creations/deletions for the month. Imagine the $ they would lose if they actually coughed up $25 every single time they create a character just so they can spam in Ishgard too.
As long as the potions are paid, not free, I could see RMT paying it every once in awhile, but if every new character got one free, you'd start seeing the RMT bots running up to the Jeweled Crozier and popping under the terrain and turning on the spam nonsense in /say by the market board non-stop all day just like we already see in ARR's areas.
Edit: Maybe a compromise would be 1 free one per new full account? They'd still be paying per potion if they wanted to do that, but it would mean lost revenue for SE, so the incentive may not be there from a business perspective. I suppose that also throws out their original intent for the potion being only for new players wanting to catch up to their friends, so I doubt that's gonna happen at all.
They don't have to go to Ishgard to do their thing. Like I said before, they still get in my way all the time.
If RMT is a problem, SE should deal with it, not charge players an extra fee. By your logic SE should also charge for each new character, maybe they should also charge a fee every time you enter a city? Maybe each time you get a load screen? That's not how you fix a problem.
As someone whose friends fall into the category of "Friends I can't get to play the game because it's too much work to catch up and I have too many things to do to dedicate all my time to helping them", I can't wait to see the Levelling Potions. It might actually breathe life into my dying guild again if the people that took breaks for various other games could come back, pay some money to catch up, and be able to play with everyone again. Yeah, we'll still have to teach them how to play their classes, but with the combat revamp, I suspect we'll all be relearning our jobs in Stormblood to a large degree anyway.
No one's saying the pay wall will stop the RMT problem altogether, but free jump potions would easily make it worse for everyone. I feel like you're blowing it a touch out of proportion there; these jump potions are optional items, and will be categorized as such in the cash shop. They're a far cry from anything actually necessary to play the game such as character creation/entering a city/load screen. Yes, the RMT problem should be fixed, but until an actual solution is found to stop it altogether, there's no point in spreading the RMT problem for everyone else in the mean time.
Thinking it over some more, I could maybe see 1 free jump potion for someone making an alt where they already have a character with a job at the level/MSQ point that the jump potion brings you to. Although I think someone already brought up the potential issue with PVP rankings regarding that. There's definitely multiple scenarios to consider before outright throwing them out there for free in any capacity. The paywall may be inconvenient, but it does at least mitigate some of the potential issues these potions may introduce to the game.