The underlying physical media may be four separate links, but at L2
it's a single 400Mb/s aggregate. If it were split up into, say two
1) assuming that costs favored intra-building connections, one of the
aggregates would be selected for pruning by the spanning tree
2) assuming that costs favored having both aggregates in service, if
utilization on the two aggregates was 50% on (call it) A and 100% on B,
the 50% available on A would be wasted. Note that latency would go up,
because spanning tree would have pruned some intra-building link would
have been pruned in order to keep the inter-building link active.
As the risk of belaboring the obvious, of course there are issues of
reliability and cost-effectiveness, only some of which are technical.
With respect to economics, for instance: The cost to link two ports in
the same building (better yet, the same room) is essentially the FDDI
cable. The cost to link two ports over a wide area is that of a
DS3/OC3. L2 is not all that different from L3 in that once you've
spent real money on a circuit, you want to get as much use out of it
as you can.