Because, when Reigns gets drafted to whatever brand he gets drafted to, he will take the WWE Undisputed Universal Championship with him, and he will stay.

Source: https://411mania.com/wrestling/aceros-wwe-raw-review-4-24-23/

How are we, as fans, supposed to believe Zayn can beat Reigns for the Undisputed WWE Universal Championship at Elimination Chamber next Saturday when they’re already teasing the Tribal Chief’s title defense against Rhodes?

Source: https://nesn.com/2023/02/will-wwe-pre-wrestlemania-juggling-act-hurt-sami-zayn-cody-rhodes/

Roman Reigns defeated LA Knight, retaining the WWE Undisputed Universal Championship with help from the Bloodline at WWE Crown Jewel 2023.

Source: https://www.forbes.com/sites/alfredkonuwa/2023/11/04/wwe-crown-jewel-2023-results-roman-reigns-beats-la-knight-who-never-had-a-chance/

Roman Reigns passed the insanely impressive 1000-day mark as Champion, and he doesn't show any signs of letting his Undisputed WWE Universal Championship go anytime soon.

Source: https://comicbook.com/wwe/news/the-usos-crash-roman-reigns-1000-day-wwe-smackdown-celebration-solo-sikoa-makes-his-choice/

What this tells me more than anything else is that WWE has already decided that Roman Reigns is holding onto the WWE Universal Championship until at least WrestleMania 40, barring injury.

Source: https://www.cagesideseats.com/wwe/2023/4/29/23704417/wwe-stock-report-apr-2023-draft-results-cody-rhodes-omos-free-agent-styles-return-bad-bunny-profits

While the Universal Championship has undergone a few pallet swaps, the WWE title has remained unchanged since 2014.

Source: https://comicbook.com/wwe/news/wwe-undisputed-wwe-universal-championship-change-triple-h-raw-report/