Why aren’t front parking sensors fitted as standard? - the same question could be asked of many other features fitted to cars within a model range - not just the Leon, but any model range built by a car manufacturer - e.g. intelligent cruise control, LED headlights, automatic climate control, auto folding door mirrors, keyless entry / keyless start, reversing camera - the list goes on.
It‘s a way for car manufacturers to differentiate between different trim levels within a model range, and enticing prospective owners to upgrade to the next model up in the range to get all the features they want / need - or if specific options can be bought separately - for customers to pay extra and buy ‘must have‘ options. It‘s also a way to make a specific model range of cars more affordable to a wider customer base, so customers with a lower budget can buy / own the entry level car in a model range, but they’ll get a less well spec’d car with fewer features.
It’s a fine balance for the customer between affordability and must have features, and the car manufacturer’s solution is - and always has been - to have a range of models with different trim levels and features that caters to as broad a customer base as possible to maximise sales. So if a specific feature such as front parking sensors is a ‘must have’ and a car of a certain trim level within a model range doesn’t have them, then the customer has to make the decision to either pay extra for a more expensive model that is fitted with them, or sticking to their original budget and living without them.