I see that in many Animes, there are cross-dressing (or maybe trans) characters, like Hideri Kanzaki of Blend-S, and they usually have awfully high-pitched voices. These characters in anime and manga fall into a trope and are usually referred to as "trap characters."
Why do these characters even have high-pitched voices?
I can't think of any character that falls into this trope and has their voice fall into a lower, typically male, voice register.
The whole point of the "trap" archetype is to trick people (both the audience watching the show, and the characters in the show itself) into thinking the character is female when they aren't. If they were to speak with a masculine voice, the illusion would be ruined the moment they opened their mouth, which kind of ruins the point.
Aside from this, it would also be very jarring to hear a masculine voice coming from someone who does not look very masculine. The reverse is also true: it would be extremely jarring to see a muscular male speaking in a high-pitched soprano voice.
Screwball comedies like Pop Team Epic may use ill-fitting voices for intentional comedic effect (see also: Up), but with "traps", the comedy instead comes from people mistaking them for the opposite gender. Giving them a masculine voice would, as I've already noted, ruin that particular gag, and then there's not much point making them a "trap" in the first place.
Because they are mostly voiced by women. In the rare cases they are voiced by men (for example when a guy has to dress up for comedic effect, like in Working) they want to pass off as women, so they use a higher pitch to maintain the deception. Sometimes this works and sometimes it hilariously fails.
If you are wondering why they are voiced by women, it's because the trap genre's main audience are men. It's a type of gap moe. A male voice would ruin this by making the real sex obvious.
