Why the fascination with subwoofers?

Some peoples speakers may require augmentation of the lower frequencies e.g. bookshelf/monitor speakers

They can be more affordable than upgrading to a speaker that has comparable bass perofrmance .

My case precisely. I like very few speakers but fell in love the sound of EgglestonWorks speakers but could only afford the Fontaines, not their bigger brother the Andras. The Fontaines require LF augmentation, the Andras, with their 12" woofers, do not. So I did what I could - added a pair of 12" powered subs, moved them to optimal positions within the room for smoothest bass response ... and voila! Andras sound on the cheap (relatively).

And contrary to the OP’s opinion, they (ACI Titans) perform very well, were not costly (relatively), and were not difficult to integrate.

very very few speakers put out 20hz bass at realistic volume levels 10 feet away; even if they are rated at low bass levels, that is at inches away and at very low volume...subs add a lot, not just bass, but make a huge difference to mids and highs...they can be difficult to place and integrate, therefore much debate and discussion...my speakers are rated at -3db at 29hz, and a single REL S/5 SHO has made a great difference, though my room fortunately made for easy placement of a single sub...

One problem with full range systems is that standing waves often exist in the listening room. This can mean bass in most areas except the listening chair. The solution isn't room treatment, its placement of several subs in such a way as to break up the standing wave.

The Audiokinesis Swarm is thus one of the best sub systems made.

My speakers (Classic Audio Loudspeakers T-3) go down to 20Hz flat no worries. But I still have to employ subs to break up the standing wave else the bottom octave is not audible at the listening position although it certainly is everywhere else in the house. 

2psyop

why add a subwoofer to an existing audio system when they don’t always perform well, are costly, and are difficult to integrate with the many varied speakers offered.

Why indeed? I sure never would. That would be crazy. Instead I would add one that performs well, is affordable, and integrates well.
See what you did there? Built a straw man in the guise of a question. Don’t do that.

Additionally, why wouldn’t any audiophile first choose a speaker with a well designed bass driver designed, engineered and BUILT INTO that same cabinet?

Many do try to do just that. Look around however, it quickly becomes very obvious the hardest most expensive thing you can find is two quality speakers with true 20 Hz bass. They essentially do not exist. Turns out (read on) that for reasons of physics they cannot exist. Which is why they don’t.

If anyone’s speakers were not giving enough tight bass, why wouldn’t that person sell those speakers and buy a pair that does have tight bass?

Well now you’ve confused tight bass with extended bass. Tight responsive bass is not all that hard. Tight responsive deep bass, that is so hard it borders on impossible. In fact it turns out that thanks to the particular combination of physics and human hearing that rules the lowest frequencies there’s really only one good way of doing it.

Also your question ignores the fact there’s more to music than low bass, and a lot of people love the way their speakers sound, and just wish for that plus really nice solid low bass extension.

Add to that, the cost of adding a really good distributed bass array is far less than a new set of speakers, at least ones that will get you anywhere near as good bass response.

Finally, they say you don’t know what you’ve got till its gone. In this case though its more you don’t know what you’ve been missing till its there. So its the unknown. Nothing more fascinating than that.