There are few observed high-mass X-ray binaries (HMXBs) that harbor massive
black holes, and none are likely to result in a binary black hole (BBH) that
merges within a Hubble time; however, we know that massive merging BBHs exist
from gravitational-wave observations. We investigate the role that X-ray and
gravitational-wave observational selection effects play in determining the
properties of their respective detected binary populations. We confirm that, as
a result of selection effects, observable HMXBs and observable BBHs form at
different redshifts and metallicities, with observable HMXBs forming at much
lower redshifts and higher metallicities than observable BBHs. We also find
disparities in the mass distributions of these populations, with observable
merging BBH progenitors pulling to higher component masses relative to the full
observable HMXB population. Fewer than $3\%$ of observable HMXBs host black
holes $> 35M_{\odot}$ in our simulated populations. Furthermore, we find the
probability that a detectable HMXB will merge as a BBH system within a Hubble
time is $\simeq 0.6\%$. Thus, it is unsurprising that no currently observed
HMXB is predicted to form a merging BBH with high probability.
