Maybe because IBM connected the 8087's INT output to the 8086's NMI, so the 8087 became a source of NMIs (and with really trivial stuff like underflow/overflow,

Maybe because IBM connected the 8087's INT output to the 8086's NMI, so the 8087 became a source of NMIs (and with really trivial stuff like underflow/overflow, forced rounding etc., to boot). Even if you could disable the NMI with external circuitry, that workaround quickly becomes untenable, especially when it's not fully synchronous.