Roasted vegetables taste different from boiled or steamed ones by a wide margin, and the difference is created in the first few minutes rather than gradually throughout the cooking.
Browning cannot start until the surface is dry
While a surface holds water, its temperature cannot rise much above the boiling point, because the energy arriving is spent turning that water into steam.
The reactions that create roasted flavour require temperatures well above that, so nothing happens until the surface has dried out completely.
Everything that keeps a vegetable wet therefore delays flavour development, and time spent steaming is time not spent browning.
A crowded tray steams instead of roasting
Vegetables release a great deal of water as they heat, and that water has to leave the tray as vapour rather than lingering around them.
Packed pieces trap the vapour between them, holding a humid layer against the surfaces and keeping them at boiling temperature for most of the cooking time.
This is the single most common reason home-roasted vegetables come out pale and soft, and no increase in oven temperature fixes it.
Oil transfers heat where air cannot
Hot air is a poor conductor, so the parts of a vegetable not touching the tray receive heat slowly and would brown much later than the underside.
A thin coating of oil fills the microscopic gaps and conducts heat evenly across the whole surface, which is why oiled vegetables brown all over rather than only where they sit.
Too much oil pools and effectively fries the base while the tops remain in dry air, giving an uneven result from the opposite direction.
Sugars concentrate as water leaves
Water loss concentrates the sugars already present, so a roasted vegetable is measurably sweeter per bite than the same vegetable raw.
Some of that sugar then caramelises at the surface, adding the bitter and nutty notes that distinguish roasting from simple drying.
Vegetables with more starch than sugar need longer, since starch has to break down before there is much sugar available to brown.
Size and shape decide what you get
Small pieces have more surface relative to their volume, so they brown extensively but can dry through before developing a tender interior.
Large pieces retain a soft centre and take longer, which suits dense vegetables and is unsuitable for anything that collapses.
Cutting for a flat face increases contact with the tray, and that contact is where the deepest browning always occurs regardless of what else is done.