Only because it's treated differently. Treat spells the same and no slot is used unless you use your action.
Or treat the other actions the same as spells, and the Action In Combat is used on your own turn.
They did if you bother to read the section. The examples of non-spell readied actions have the FULL ACTION happening AFTER the trigger. The wording says that the ACTION (not partial action, because the rest is done) happens AFTER the trigger.
Why point out the section, and then misquote it in such a fashion that your misquote makes it seem one way when an actual accurate quote shows that it works the opposite way? Did you think I wouldn't bother looking it up?
You use your Action In Combat, on your own turn, to take the Ready action. This doesn't give your permission to take a different Action In Combat as a reaction; it lets you
act on your reaction.
The 'act' you take may or may not resemble the thing a specific Action In Combat would give you permission to execute. In the examples given, one of them is 'pulling a lever' to open a trapdoor. But you don't need an Action In Combat to pull a lever on your own turn because you can pull a lever as a free object interaction. All the Ready action allows you to do is to pull the lever when it isn't your turn, in response to a trigger.
Here's an example where the distinction between 'Action In Combat' and 'act' matters: I have the Shield Master feat, and if I take the Attack 'Action In Combat' on my turn then I can use a bonus action to shove with my shield. I can execute the shove before I execute the attack (confirmed by JC). This is because it is not the actual execution of the attack that generates the bonus action, but simply 'taking the Attack Action'. I can take the Attack action now, and actually execute that attack at any time from 'as soon as I take the Action' to 'the end of my turn'. Once I have taken the Attack Action and the bonus action generated by taking it, I can execute the attack and the shove in any order I like at any time until the end of my turn.
How this matters is that I can Ready the Attack Action on my turn, which includes 'taking the Attack Action' on my turn (generating the shield shove bonus action), but the Ready action allows me to execute the attack allowed by taking the Attack Action later in the round in response to a trigger. Meanwhile, on my own turn, I can execute the shield shove.
The sequence of events is this: my turn starts, I take the Ready action to use the Attack action. At this point no attacks have been executed, but I now have generated a bonus action shield shove (because I took the Attack Action as part of the Ready action), and I have permission to execute that shield bash at any time during my turn, and I have permission to execute a single weapon attack, using my reaction, in response to a trigger.
If the Ready action could not also include taking another Action In Combat as part of taking the Ready action, then you wouldn't have permission to do anything on your reaction if that act would normally require an Action In Combat to execute while in combat rounds. If Ready
forced you to do something that requires an Action In Combat, then you couldn't do things like pull levers or say a phrase.
If the act that you want to take in response to a trigger is an act which would require an Action In Combat to execute, then you take that Action as part of taking the Ready action, and this happens on your own turn. Without the Ready action, you would only be allowed to execute that act during the turn you took the Action that allowed that act, but Ready allows you to execute that act in response to a trigger and allows you to execute that act when it isn't your turn.
The Ready action does not let you 'take an Action In Combat'
per se as a reaction; it lets you
execute the act as a reaction. In colloquial English, we don't say 'execute the act', we say 'perform an action'. But in this case 'perform an action' means 'executing an act' rather than 'take an Action In Combat', since, by RAW, you can only take Actions In Combat on your own turn. Ready doesn't change when you 'take Actions In Combat', it just changes when you execute the act allowed by that Action In Combat that you took on your turn.