1- they start by owning all validators, maybe they expect to open validators to other entities at some point in the future. If these entities don't collude together, we could expect some sort of neutrality

2- Marketing - because crypto is coming at an ATH and why not getting some good marketing for free (or almost)

And people mentioning costs, this is not particularly relevant. L2s are extremely cheap by most standards, let alone by Stripe standards which charge horrendous fees.