I'm not in the public sector, but I've seen this a lot in the enterprise world.

That is the problem with how organizations prioritize capex over opex in their budgeting. Most teams I've worked for can request a bigger capex budget for a project because it is "one time". Even though they ask for a bigger budget every year.

But asking for more annual opex is a harder sell on the bean counters, because the increase of operating expense of a department is seen as a permanent thing. It only goes up, never down, so lets just not give them money for it.

This. Also capex and opex have different effect on taxes, since the government usually wants to stimulate innovation (subsidize capex).

You're exactly right. Officials like to award contracts for new systems--it makes them look important, it gets the systems integrators to kiss up to them, and the failures won't become apparent until they've moved to their next job.