Why do so many things cook at 180C/350F
This is as hot as you can get without quickly burning any sugars or cooking fats on the surface of the food, so it is typical for enriched breads, cookies or cakes (which have plenty of sugar and fats); note that doughs containing not much sugar (bread, pizza, savoury pie crust) are sometimes baked at substantially higher temperatures.

Non-pastry/bread dishes are often kept at 180°C or below because the above is also true for vegetables and fruit naturally containing sugars, resulting in much accelerated charring if going above that temperature range.

The difference in browning in a rich dough (eg streusel) between 160°C and 180°C and 200°C is drastic.

A still oven that is truly at 180°C (by oven thermometer) can already be literal few degrees too hot for recipes calling for 180°C; an oven that is out of calibration even 10°C hotter can cause a lot of recipe failures.

There seems to be a general consensus that non-still (fan assisted/convection/..) ovens should be set 20°C lower (if the fan/convection is used) than you would set a still oven. This is commonly found in recipes and manuals.