browsing around there seemes to be a lack of a good selection of active 3/4 way floorstanders.
Why arent active speakers (other than monitors) common
There are actives for 20k but where are the 5k ones ?
Should I just spend 2k on active XO's and amps and build them myself ?
I think you are over optimistic. I don't think a 5K active speaker would be much good. I think you need to be in the 10 to 20K range at least. If you want good active three ways you need to look at ATC who make a range of really fine active speakers for professional monitors and domestic loudspeakers.
I think if you have never designed and built a speaker before, the chances are you will fall on your face with a very bloody nose by starting with an active three way. Even if you did you will not come out for 5K.
Certainly you need to have experience of design with some simpler speakers first. You definitely need to start with a two way. You will need a capital outlay for speaker design software, tools and measuring equipment. If you go about it the right way there is nothing to stop you becoming an experienced and skilled designer. But you have to gain experience and it is all great fun and certainly not to be discouraged.
Now in an active speakers you still have to design the crossovers to the speakers acoustic responses.
Certainly there are advantages to going active, and for some designs in my opinion it is mandatory.
At crossover points below the 400 to 500 Hz range I think active designs are mandatory. Even in that range I think an active design is to be preferred. In the higher ranges, the advantages are less clear and there are certainly downsides. For one thing it is a really good way to blow tweeters if you don't include tweeter protection in the design. Even a small turn on thump is death to a tweeter.
So there is a lot to be said for hybrid designs where the lower crossovers are active and the higher crossovers passive.
If you can get a few good two way passive designs under your belt, then you are ready to may be undertake the design of three way speakers and active designs.
If you think you can buy off the shelf crossovers, amps and drivers and easily cobble together a first class speaker or even one that justifies the investment, then you are mistaken.
Certainly your aims are on target, and I agree that there should be more active speaker choices than there are. However few are able to justify the cost.