The terms of service for online play is effectually a social contract, in which players mutually agree to follow the rules. By following the rules, players agree to not cheat, such as using external applications to read/write data, either directly or indirectly.
Don't undersell this advantage. What can you gain by saving time? More time to practice? More focused practice? More varied practice? Nobody has infinite time, so saving time by cutting corners gives you an advantage over others who do not cheat.
It's explicitly disallowed by the rules / TOS, just very weakly enforced.
Correct, but they are breaking the rules and are occasionally caught/disqualified for doing so.
Because they prefer to follow the rules; the same social contract that every other honest player has agreed to.
Not true. Many such examples of things people mistakenly believed as legal at the time/lack of knowledge (such as Sejun Park's Magmar or Wolfe Glick's Moltres, or Shohei Kimura's Amoongus etc.) are actually not legal, and give advantages for their unnatural stats. The official hack checks are not all-knowing, and many things slip through.
Let's keep this only related to cheating in supposedly "legal" things. An aside, you can inject wildly hacked things into any game you want, so your statement is factually incorrect. Usually the official hack check will prevent you from trading/battling with them.
Social contract, agreeing to not cheat. By agreeing to the rules then violating them, it should be obvious why there are people that get annoyed by those who do rule-break.
It's a selfish AND elitist mentality to think your time is worth more than others' time, in that you deserve to disobey the rules you agreed to in order to compete. By being dishonest to your peers who agreed to use it, are you really the one in the right? If you don't like the rules, then don't agree to them and don't play in tournaments/ranked. Only play against others who agree with YOUR definition of acceptable rules.
Welcome to the world of gaming. Video game companies exist to sell games and services, and they run promotions/events/circuits in order to market their games. They don't do it for free; they want you to buy their games and spend money on add-on transactions like DLC. You are the consumer, and ranked/VGC is their advertisement of competitive play. One might argue that generating mons increases accessibility and lets more players play with optimal teams, but that is against the rules. The game company calls players "Pokémon Trainers", not "battlers". They want players to play the game, and keep the online community vibrant by trading and battling in-game, so that others are more incentivized to buy the game and join in. Not for players to battle on simulators or receive hacked teams, and never engage with other players besides battling with perfect teams.
You'll surely have fun when the upcoming Switch-2 games slam the door shut on hacking/injecting Pokémon. Better get used to obtaining teams legitimately.
Don't like it? Play another game