So did 1st Succession War.
To reflect this homogeneousness, in battles set between 2786 and 2807, once the weight class of a particular lance has been determined, roll only once on the Random Unit Assignment Tables per lance; all ’Mechs in the lance are of the resulting type. In games set between 2808 and 2819, roll twice for each lance. From 2820 onward, normal rules for Random Unit Assignment apply.
As an aside, also from 1st Succession War (p 142):
Finally, remember the two “prime directives” of playing BattleTech:
1. HAVE FUN
2. DON’T LET YOURSELF GET SO CAUGHT UP IN THE RULES
THAT YOU STOP HAVING FUN
So basically, it IS reflected in the rules - but really, fun is more important, and a mixed force is always going to be more interesting than a homogenous one. It's not like we're throwing entire regiments at each other, where you'd expect to see more variation. Game scale makes it small-unit focused, though I suppose Alpha Strike could easily handle much larger, homogenous forces at a 'realistic' organization.
And in all honesty, even in modern militaries there's always mixed forces, especially as technology and lethality improves. Look at an infantry battalion combat team, with attached weapons companies, each line company with weapons platoons, artillery and a glob of armor in support. It's not THAT unrealistic, really, if you play with the ideas a bit. Hell, even a modern rifle platoon has a mortar or two, a few machine guns, a couple dedicated grenadiers, and a dedicated command unit mixed in with all the rifles.