Bit late to the party on this one 😉
The short answer is they already do. Gorechosen, Elector Counts, Fireteam, Bladeborn, Doomsday, Lost Relic & Blitzbowl are all examples of games produced that I would consider "game in a box" where you pick up the boxed set in a shop and have everything you need to play it. Sadly many of them are region locked for reasons I've never fathomed
What we have seen is GW struggle to keep on top of everything they produce though. Cardboard production has been a thorn in their side for many years now as much of it is done in China with the horrific logistics issues we've seen in recent years. That's resulted in GW pulling back to focus on things they can support & manufacture in house.
Board games that have long term support (and purchasable in GW stores) you've largely just Underworlds and Bloodbowl. Both work out the box with the teams in them, but have support mostly through miniature releases. The new Rivals format introduced a couple of seasons ago in Underworlds means that you don't need to keep up to date with every item being released now.
Would I like to see more board games? Absolutely! I've Horus Heresy, Space Hulk, Dreadfleet, Gorechosen & Talisman at home which are all excellent games without the worry about being a brand new game system to learn. None of them have a requirement to paint the miniatures, nor do I need to purchase a mountain of miniatures to play the game fully.