I agree the real reason why they are not joining is political. Just like VW exit was.
For Skoda it's about 60% the money related to coming investments (and the PR/shareholders effect if they use large sums on WRC at same time). The rest is related to the current rules cycle coming to an end in 2-3? years. The new rules definitely need to include some hybridization else WRC ends like WRX did. Since it takes 1-1,5 years+ to develop a winning WRC it's late to join now.