Back in the day, every videocard manufacturer released their own drivers. Diamond, Canopus, BFG, etc, they all took the baseline GPU drivers and added their custom branding. This meant that after Nvidia released a new driver, it could take weeks before Diamond put it up on their website. You missed out on bugfixes and optimizations for new games. So everybody just used Nvidia's drivers, and eventually all the various companies stopped doing it entirely.
Mobile VPN apps are in the same spot. They all use the same backends; OpenVPN, IKEv2, and/or Wireguard. Everybody's time would be best spent making _those_ apps better. The OpenVPN iOS app has a particularly poor UX.