As Jarvie nicely explained correctly, because you're making an operation on the chain that is picked up by the API responsible for notifications within the HIVE node software.
When you hit that button, it sends an on chain signal that is picked up an processes by a witness node somewhere, you're causing computing to happen within a node on the network an in order to prevent functions from being abused to negatively effect network performance you need to make it have a cost, else there is not much in regards for protecting the network against bad actors making 1000 accounts all spamming free functions that need processed by whatever current node is processing blocks.
It's a security feature in a sense, and the actual cost of the call is miniscule provided you have more than like 10 HP.
This isn't a Web 3.0 implementation captain.. That refers to the Ethereum based technology, metamask and whatnot. HIVE is graphene based.
As someone who runs a node that HIVE runs on I'm glad it's not a free for all, because as important as user experience is.. If the network itself just was wide open to calls with no cost involved RC wise it would be trivial to DOS / DDOS attack nodes. RC exists simply to make it economically unfeasible to do this easily.. As you can't just spin up a bunch of new malicious accounts with no stake and cause havoc with them. Very thankful for this
If you're honestly in this ecosystem and enraged that your account that is effectively unfunded and free can't do a whole bunch of things due to lack of RC the ways around that are to stake HIVE in Hive Power to unlock a greater mana pool.. or ask someone nicely to delegate you some Hive Power / network bandwidth, which both myself and giftgiver have done at this point so you aren't hitting the limits.
New accounts with new delegation are set up with juuuust enough RC to do a few operations a day in order to not be a weakness or attack vector basically, or rather to make it not feasible time wise to do so.
I do getwhat you're saying though as a deterrent thing.. Because in a sense it is designed to be a deterrent 100%, so you are right. It just shouldn't be one that is felt by users not abusing the system, and that is where the social nature of this network kicks in that a post such as yours will get the community to throw more RC at you.
It ain't perfect, likely needs tweaking, but it serves it's purpose.
Also note, most things that have a client have a corresponding server that the client connects to, depending on what it is you are processing sometimes it's better to have it client or server side.. While doing it client side is totally possible, it might end up making a nightmare for low processing power devices.. Can you imagine an early iphone or ancient laptop trying to index through thousands of post entries and date checking every time you open a page or go to view trending? It would make it completely freeze. :P