Because raiders require the most stuff in their hotbars as a lot of very situational abilities/items become worth using? There is no point in having your entire sixth bag on your hotbar, stop being so damn lazy and just open the bag for the item you will use once every week.
Here is a cool video howto create an 11th flexilbe DaqUI hotbar with 10x10 macros https://www.patreon.com/posts/tip-of-week-39945384
I just saw this post and omg..NOOOO. If anything, do we need all these different spells? Couldn't they just increase the damage/heal/cure/whatever on some of them and remove some to get the same effect? My screen is already so cluttered I hardly have room to see where I am going. 10 hotbars, and all the other windows that have to be open. ugh !
They've been asked over and over for 10+ years to revamp combat abilities and consolidate them. It's been pointed out, time and again, that there are far too many active abilities. They have ignored this feedback for over a decade so I don't see it changing.
Every class has "filler" abilities. These are usually kinda meh direct damage stuff you just spam when your important stuff is cooling down. All that can be squished into one macro.
It's not just the spells. Every class has both profession spells/CAs and Ascension spells that are practically worthless and that they just don't use, so I start by kicking those off the hotbars. But I raid, so I need a bunch of cure, heal, power, and buff potions/baubles on a hotbar. I need 5-6 hotkeys for snacks, which last 10 minutes and cannot be placed on autoconsume or stacked in a macro. Since things do debuff you, I have a hotbar which contains macros, each with two of my persistent buffs, so I can visually glace at it and see what needs to be recast. I have three mount/familiar setups, so I have three hotkeys that equip/summon the mount and equip the familiar, and a fourth key that summons the familiar (they can't all be in the same macro). I also have runspeed totems, evac totems, tinkered items such as Call of the Tinker, the one that acts as an evac, and the mender bot. Then there's CoH. And I have a couple of social macros, one for bidding on loot in raid, one for LFG. This is all on top of my profession spells and Ascensions. DarqUI has an "almost hotbar" that you can define a key for stuff like equipping Hadden's earring, using it, then putting your original earrring. Or using a bauble in your bags. etc. But that doesn't work quite as well as a hotbar.
Tip: Use DarqUi Settings macro for the snacks .. with an additional ACT trimer for reminding you after 11 minutes Autocosumption if intellect potions and totems breaks so often that it's almost useless atm