Again, if you think you’re not missing something, look the gear of raiders and see how much is from M+. If gear is so plentiful in raids, it should be nearly all raid gear.
Let’s take a hypothetical average 9 hour per week raider that somehow does not do M+, and an average 9 hour per week M+ player that does not raid. Let’s also ignore the first couple weeks when mythic raiding is not out, and people are leveling keys.
First, let’s assume that raider has filled every slot with current tier heroic and normal raid gear. They are about 230 ilvl
The M+ player, with valor upgrade and vault, is going to be about 235.
The average raider, didn’t get 3 bosses down the first week and only 3 people in the raid got loot, so they are still averaging around 235 item level. The only upgrade was a stat upgrade from their vault.
The average M+ player got their 252 from the vault, and have upgraded through valor. So they are closer to 245.
The next week the mythic player got no loot but got 3 bosses. So their item level is now 242.
The M+ player got his 252 and upgraded again with valor he is 247.
Continue this pattern for a few weeks.
The raider has seen at least the first 4 bosses in mythic. With average luck, they have seen 3 or 4 pieces of gear, and probably only duplicated 1 piece of gear in their vault. So they are now at 5 or 6 pieces of 252 gear, but because their secondary gear is still 239 they are only around 245 item level.
The mythic plus character has been able to pick from 2 or 3 pieces of 252 gear per week, so with average luck, have not overlapped any slots. The only pieces they upgraded are those that were best in slot and only replaced 1 or 2 of those. So the mythic+ player is sitting around 249 item level.
But here is where M+ really takes off. The raider is doing the same few bosses every week. They have everything useful off the first 2 or 3 bosses, but they keep getting loot off those bosses. Worse, they only get 1 or 2 choices from their vault, which is consistently from those bosses they don’t need loot from. The M+ player is getting their 3 choices, and those choices pull from an item pool that is literally everything from any dungeon. Even if they didn’t do that dungeon. Even if they aren’t getting an item level upgrade, they are at least getting a stat upgrade, when the raider is just seeing the EXACT SAME item.
It isn’t until (really if) a raider starts pushing CE and opens up ALLLLL options from the raid that they can pull ahead of M+. This is the top of the top players who have invested 10 hours+ per week over the course of months, for maybe 3 item levels above what a M+ player could do with less than half that time investment. The number of people who clear the whole mythic raid every week, rather than push the content through raid lockout extensions (which gives zero vault loot), is negligible.
EDIT: Hadn’t seen this post before I wrote this. If you want to see people honestly speaking from the other side of things try this post: Raids are disheartening