Better players than me can explain but:
Land is Slow. So making deploying a land army in mass takes alot of time or transports
Firebases. Or more specifically A failed T3 Engagement can backfire so painfully
Map Control is Generally establish, so engaging on fronts to take proxies or expansions is harder and might take more mass than you give
And likely also efficient usage of BP, creating and deploying the Bricks/T3 Army takes more time/effort/etc to build than just amassing T4 Units or Arty. So even if equal mass of bricks > ML or otherwise. The relative time to make, deploy and otherwise the bricks is harder than the monkey mayhe/IDunno. Shitty 1k global