Let's get this out of the way, Hi-Fi speakers are a luxury item. Anyone willing to pay over 4-5L for a speaker is certainly doing it not for the utility, but for the brand value, the perceived 'Audiophilia' and the prestige it brings in the social circle. There are several world-class brands that offer full range speakers of various sizes and qualities, not to mention prices that range from expensive to eye-watering.
What home theaters aspire to do is recreate the 'immersive' experience that you feel in concerts, stadiums and movie halls, as though the experience has enveloped you. All advancements in sound technology, acoustics are geared to bring you closer to the experience that was originally present either in the concert hall, the recording studio or the live setting in which the sound was recorded.
Once we accept that Hi-Fi speakers are luxury goods (albeit electronics), we can focus on the specifics of what make great speakers great. Some of it is down to the material used within the speaker, the R&D and proprietary technology in the drivers/woofers or electronics. To the discerning listener, it is easy to distinguish hi-fi speakers from the ones you find in the local retail store, the sound is way more refined, each channel sounds clearer and the audio reproduction is more accurate.
The engineering and R&D cover all aspects of the speaker unit; the type of materials used inside the cabinet, the wood, wires used, the connectors as well as the crossover parts. The best known brands in the speaker industry are also those which have patented and produced some of the technologies we take for granted today, so the investment in all that IP has to come through somewhere!
The top notch speaker brands usually have invested heavily (and continually) in cutting edge R&D, make their own drivers, ensure that the components that go into the speaker are of a much higher quality; cabinets are made of high-grade wood, propreitary designs are used, wiring is top notch, speaker cones made of best quality wood/metal/kevlar/poly material. All of this ensures that a well made speaker lasts ateast for 25-30 years and often the market value of a great speaker tends to slightly appreciate along with time.
There are different sound ranges, low frequency (bass), mid frequencies (human voice), high pitch (strings, subtle tones). It is generally agreed that quality speakers are adept at playing (accurately reproducing) all ranges of sounds audible to the human ear. Even so, the load of playing specific frequencies are delegated to specialist instruments; the most famous of them being the sub-woofer that handles bass (low frequency thundering/booming/rumbling noises).
Although you may assume that the largest costs in the speaker go into the diaphgram that drive the sound, it's actually the casing that take the major share, with multiple combinations of high grade wood/metal used to make the cabinet. When the diaphragm vibrates, it creates sound waves all around it, so the casing's major function is to withstand the drivers without making any vibrations of its own to avoid adding additional sound waves and make the music jumbled.
In addition, the material used to make the tweeter drivers (i.e. the drivers making the high frequency sounds) are usually made of neodymium or diamonds (these materials are light but also hard which ensures they can operate with minimum energy & withstand pressure) are expensive. The prices of the former are said to be up by almost 2000% in the past decade because of it's uses in various products such as smartphones, laptops and high end electronics.
To conclude, Let's look at this another way, the best speakers (which are engineering marvels in their right) cost as much as a small car. Yes, a small car. When was the last time you spent so much time agonizing if a small car was worth the purchase price and whether it would be 'worth' it?