Theres certainly logic to starting small, and then being allowed to unlock more, its a gameplay goal for starters, something to aim towards. It's the same as sandbox mode vs regular, why stop people from building the best items day 1? well it gives you stuff to earn.
I dont disagree that it'd be nice to be able to start with a full unlocked map, after a few fresh starts it might get annoying having to start in a small plot, but it also has the advantage of focusing your attention and tune you to sensible planning because you can just sprawl yourself across a vast landscape.
I know its been mentioned elsewhere, but hopefully we get rulesets like the CiM series had, and we'll be able to enable/disable and balance certain features to our preference. Maybe even need to unlock them through in-game achievements (complete freedom for sandbox), so at least it starts by guiding you, but as you hit certain goals (population, wealth, expansion etc) then you can enable & tweak these options within the ruleset.
I doubt they're at a point in development where they can add or change too much tbh, its probably just finish what they've got planned and fix any issues, but hopefully rulesets are carried over from CiM.