To be clear, it's the electronic components of the Vue that are all on a single wiring harness (at least since Rev E came out some years back IIRC). But the mechanical components (wind cups/vane, wind cartridges, tipping spoon mechanism etc) are all readily available as individual spares.
The other general issue is that all-in-one sensor assemblies, especially where the anemometer and rain gauge cannot be separated, are neat, easy to mount and cost-effective to produce. But all have the limitation that you cannot achieve optimal exposure for wind and rain simultaneously (you need to mount wind up high and rain gauge down low for maximum accuracy), at least for meteorological purposes. In my experience, farmers have somewhat different requirements being more interested in wind at spraying height (say 2m/6ft) than high in the air. So an all-in-one mounted at say 5-6ft might be acceptable here, though rainfall will typically be a little under-recorded and you have to take more care with finding a well-exposed mounting site - an anemometer at 6ft is much more likely to be influenced by nearby buildings, hedges etc than one higher in the air. It's also the case that not everyone feels the need to measure all parameters with equal accuracy. So some users are in practice much more interested in say wind speed than rain and so will mount an all-in-one higher, or vice versa for other users.