“Fault” is the wrong word. The question is why should the wealthy care that it’s causing grief and hardship to other families who feel they need to keep up?
The answer is: vahavta l’reacha kamocha. It’s no less a chesed or tzedoko (probably much more of one) than helping them pay for (or giving them loans for) their weddings. It’s also part of kol Yisroel areivim zeh l’zeh.
And don’t dismiss the pressure as easy to overcome, just because you Bh don’t deal with that. Even if one spouse is comfortable with a simple chasunah, the other spouse (or child) might not be. Also people feel pressured to make a good first impression and start the relationship with their new child-in-law on a positive and generous footing (and generate goodwill with their child who is moving to a greater degree of independence). Moreover many people feel that they are (and indeed they probably are) judged by their wealth (it affects their social standing) within their community (e.g. with regard to shiddduchim of future children).
Finally this is viewed by many as a particularly wasteful/frivolous form of spending. (At least a large house provides many years of benefit and usually appreciates in value over the long term).
This is not different than the takana to limit spending on funerals discussed in the gemoro, for similar reasons.
We all have to spend within our means, and if Hashem has blessed someone with wealth and they spend with a healthy ratio on whatever it may be, a house, a car, a vacation, or a wedding, it is their prerogative to do so.
The Wedding issue just magnifies the real issue within American society and our community in particular, where people live beyond their means and don't plan for their financial future, and where we feel entitled to "keep up with the Cohens".
(Granted we have a very unique social structure, where the person living paycheck to paycheck sits next to a Multi Millionaire or Billionaire in Shul or at Daf Yomi Shiur and where their children are friends and share in each others Simchos)
Placing blame on the wealthy among us is refusing to take responsibly for one's lifestyle and financial decisions.
Additionally, IMO the peer pressure to spend on simchos (or houses, or cars, or vacations) beyond one's means is not coming from the extremely wealthy, who we understand are in a different "class", but rather from the others who spend beyond their means and drive up the standard of "middle class" lifestyle.
"Also people feel pressured to make a good first impression and start the relationship with their new child-in-law on a positive and generous footing"
Is this a reason to spend beyond one's means, to make a "good first impression" by faking it? for what purpose, to perpetuate the cycle?