Ignoring the opinionated part of the title for a moment: because there's money in it, pure and simple. That money comes from the buyers, in a kind of pseudo-Ponzi scheme, but it's still the basic motivator for getting involved in ETH.
A good example is if ETH or some other ccy takes off and BTC withers. What is a BTC worth? nothing. But people are buying it because it'll be worth more in the future. That is the definition of a bubble. Given that the original owners make the money from new entrants - that is the definition of a pyramid scheme.
By that criteria, any company that undergoes an IPO is also a pyramid scheme.
I agree that the entire cryptocurrency market is in a bit of a bubble, but to write it all off as a pyramid scheme seems a bit of stretch to me.
edit: also, "pyramid scheme" to me is a very loaded term with a specific legal meaning to it as it applies to one single entity, i.e. the person or company operating at the "top" of the pyramid. There isn't really such an entity at the top of the entire cryptocurrency market, so I think it's much more apt to call it an asset or asset class bubble rather than toss around such contentious terms.
With cryptocurrencies this is driven by excess trust for the leaders in the space and for each coin. Probably driven a lot by "technology eating the world" and "young genius" mythologies.
Anything that is grown organically rather than designed top down (like USD) needs some sort of bootstrapped energy source to amass energy/capital for maneuvers. The USD uses proof of military power to arbitrarily generate capital but many altcoins only stake the reputation of their creators which many people would willingly give up for millions.
So while ponzi schemes definitely have their downsides due to lack of sufficient proof of stake, the structure is very similar to many organic methods of amassing energy. Startup funding is similar though safer because there is supposed vetting by smart people (angels/VCs)
So I actually think that ponzi structures are rather important as an organic energy source. However they need to be vetted by asking questions look like "what are you going to do with this energy?" and "what is your proof if stake?"
What strikes be about altcoins is that it's unclear how the energy will be used. It reminds me of throwing money at the TSA for ridiculous theater because too many people think throwing money at problems solves them. (an extension of military minded brute force IMO)
Once the technical fundamentals start becoming solid I'd place a bet on the coin that has the strongest marketing as the increased mind share would directly increase the stability and trust in the coin. Btc/eth may be sacrificial lanbs like pets.com as this new space is explored. (or may be Amazon/Google, who knows. )
I wonder how many people who buy ETH are doing it because they believe in the future of the tech versus just gambling (which is basically what I did). All the hacks and scams involved have kept me away from keeping any money in it long-term.
I'll be the first to admit that I'm probably the type of investor whom people will point to as an example of an ethereum bubble (someone who knows little of the technicals behind ethereum, but is in it 'for a quick buck').
It truly scares me to hear about cases where somebody invests a large portion of their worth into something as volatile as crypto currency.
I always see "traditional" investors point to this and say "How could you possibly invest money into something you don't fully understand?"
I'm personally not invested in crypto at the moment but I can understand the draw- my response is this: When you look through your 401k, how many of those companies are even recognizable? If I were to ask extremely basic questions about any of their financials, could that person speak to them? The answer is almost unanimously "No", but it's completely accepted (beyond accepted really, it's pushed on you) to invest into companies that people don't fully understand, yet shamed if someone takes a gamble on crypto.
That's why the general advice is for people to fill their 401k's with a few index funds (and chasing average market returns) rather than trying to get lucky by picking the next FB, AAPL or GOOG.