"Why is that cake so expensive?"
Translation:Miksi tuo kakku on niin kallista?
Here it is perfectly legal to write 'kallis' or 'kallista', depending on whether the speaker refers to the cake as 1) an uncountable matter that can be divided into and bought as pieces (cf., milk as a liquid in a large tank) or 2) as an undividable and countable unit that has to be bought all at once (cf., a carton of milk). In general, pieces of cake offered to be bought at the counter of a coffee shop would be the first case, unless one is referring to a single piece of cake specifically.
In the case of a single piece of cake, one has to express that meaning by mentioning the 'piece': 'kakunpala' or "pala kakkua". The word 'kakku' as such refers to both the concept of a cake and a whole cake, but not to the pieces of a cake.
One can have a crate of milk bottles, and the same applies:
• the specific kind of milk in general –> "Miksi tuo maito on niin kallista?"
• one specific milk bottle –> "Miksi tuo maitopullo on niin kallis?"
The difference to the cake is that as milk in itself (as far as I understand) cannot be countable, it makes no sense to try to refer to a unit of it using just the word 'milk' itself (unless, in some cases, the unit is implied). 'Cake', on the other hand, has a clear meaning as a unit of matter (although context-dependent, of course, and expecting that the context includes a whole cake in the first place).
• Miksi tuo jauho on niin kallista?
• Miksi tuo jauhopussi on niin kallis?
• Miksi tuo leipä on niin kallista? (A specific kind of bread in general)
• Miksi tuo leipä on niin kallis? (A loaf of bread)
• Miksi tuo leipäpala on niin kallis? (A slice of bread)
Sure, I can give examples, but I already wrote the matter out above—there is not really more to it than that.
The word 'kakku' can be both countable and uncountable, and the English version does not specify, which of those cases is supposed to be the context for the Finnish translation. In English, one can ask the same question, whether the context is a single cake (countable --> 'kallis') or some cake in general (uncountable --> 'kallista'). Furthermore, if one wants to refer to a single unit other than what is obvious (such as a piece of a cake vs. a whole cake), the unit has to be specified. The English version does not mention a piece, so it has to refer to either an individual cake as a whole or some cake as an uncountable matter. In the latter case, maybe there are many cakes or many pieces of cake and the person is not referring to any (piece of) cake specifically.