The NVSE *plugins* don't do anything in-game by themselves, right? They just extend the engine's capabilities. So what reason is there for not bundling them together?
The way I see it, it's because the plugins listed (JIP LN, lStewieAl's Tweaks, and JohnnyGuitar) are each humongous plugins of their own.
JIP LN was initially two separate plugins, jazzisparis's (JIP) plugin and LuthienAnarion (Lutana) plugin respectively. Although I can't find the old pages for them anymore, I'd assume they've been around for as long as since NV's modding scene starts growing. Lutana's then merged into JIP's, so now we have a big ass plugin with +900 new functions.
Meanwhile, the other two was only recently (at least relatively recent when compared to how old NV's modding scene is) conceived; both was released in 2019. JohnnyGuitar adds 150+ new functions, but lStewieAl; he's one hell of a mad lad. His tweak is HUMONGOUS.
In comparison, while I can't seem to find the complete changelog for MWSE in the github they linked (albeit it's still a thorough and comprehensive documentation nonetheless), the one that's up on the Nexus is a rewrite, and was only released in 2018. I'd assume they've been working on it quite a long while, and Morrowind was released 8 years prior to New Vegas, so that might explain it, I guess.
Finally, while they don't work together to bundle all these plugins, both JohnnyGuitar and lStewieAl actually thanked jazzisparis in their credits, with JohnnyGuitar having actual contribution from JIP (not sure if they actually meant the plugin or if the individual have actual personal hand in it) and lStewieAl being of one of them actively created and maintaining JohnnyGuitar.
So why won't they just bundle them all together and make one big ass NVSE like Morrowind modding did? Like Sigourn said above, fuck if I know. They could be drama queens (as NV modding scene notoriously is known for), or they might have their own problems or life to look out for, so they can't handle such big project atm.