Local gov doesn't have anything like the budget of central so they don't typically have the choice to build their own. Instead they're left with cobbling together behaviours from tens, maybe even hundreds, of disparate third-party systems, each independently solving the full stack of a given department's function. E.g. there's a dedicated system for planning approvals, another for administering blue badges, etc., etc.

These third-party systems are typically very cheaply made, with a very low maintenance budget, and don't provide anything like enough customisation to adhere to someone else's design system. If you're lucky you can add your own header/footer and set some main css colours to match your council's brand.