Because mythic raids overall are the hardest. Thays why you get 285 ilvl loot from final 3 bosses, guaranteed 278/285 in vault, and on average 2 278s and 1 285 on a full clear. Making it the most lucrative way to earn 278s as well as only way to earn 285s.
M+15s is the next best gearing. Doing 1 a week gives u 1 278 piece in the vault. Id argue 15s are roughly the same difficulty as early heroic encounters. This gives heroic raiders a good opportunity to get a big vault upgrade. Its also the reason m+15 drops 262s compared to 265 from raids. Valor is capped first half of season and allows u to gain a 272 every 2 weeks from upgrading a 262 from the m+15.
The lower m+ are tied with normal raids which is why their ilvls are lower.
Then at the bottom are wqs and lfrs which is why they have the lowest ilvl.
If you're gonna argue that wqs should give highest ilvl, then what's the point?
Solo activities will never be as rewarding as group activities in an mmo.