From limited experience I have noticed that x-overs use both large C & L values. In those manuf that do not believe in biamping &/or biwiring, these manuf have used the combination of the large L & C value to effectively resonante each other out so that, at the input to the speaker, the amplifier sees a (mostly) resistive load.
Now, if you split the low & high x-over ckts, the large L no longer sees the large C & the large C no longer sees the large L. Thus, in the bass region the amp has to drive a large L & in the highs the amp has to drive a large C. Both of these conditions could render the amp unstable & the speaker manuf would rather avoid this situation.
Now, if you split the low & high x-over ckts, the large L no longer sees the large C & the large C no longer sees the large L. Thus, in the bass region the amp has to drive a large L & in the highs the amp has to drive a large C. Both of these conditions could render the amp unstable & the speaker manuf would rather avoid this situation.