Seijitsu's answer refers to word of god, so I won't go that way.
I will assume the OP is male since his profile does not exist ATM.
Why go to the trouble of doing a nail job if it is going to be under a glove is the same thought that makes me pull the first boxers out of the drawer and put them on without even knowing its color. Same goes for socks most of the time.
Yes, I am also male. But the other gender works differently.
Girls wear pretty lingerie nobody will see. It does not matter if their wonderful nail job is hidden behind a glove, they know the nail job is there and they feel good with themselves because of that.
Or if something might happen later in the evening. Narusegawa of Love Hina did that: in episode 5, she has to sleep in the same room as Keitaro. It did not matter that the poor boy was bound and gagged, she got the nice lingerie "just in case".
The show is aimed at young girls. Tsukino is 14 when the series begin. The target demographic is going through puberty, beginning to feel like adults. Even though most of them already know how to do makeup (link link), they are at the age their bodies are changing shape.
The showmakers are aiming at increasing identification between the protagonists (Sailor Senshi) and the viewer (young girls). The principle at work is the same discussed in this other question.