A few reasons:
1) The cost to build a modern experimental doesn't pencil out on a resale basis compared to the fully discounted acquisition price of a 40+ year old legacy spam can 4/6 seater. Which is why I was so hot after the primary non-commercial category (and of course the FAA snuffed it). RV-10s on a resale basis make no sense compared to a used up SR-22, Saratoga SP or F33C. Granted, I'd take the RV-10 any day based on the maintenance and parts allowances alone.... but not for housing money, which is where both the RV-10 and the SR-22/Toga/F33C live.
Builders are of course, generally insensitive to that argument. I personally don't like that litmus test, which is why I have a bone to pick with the 51% rule, but that's for another day. In a perfect world (and that's where MOSAIC comes into play), EAB resale owners would have the legal ability to inspect their personally-owned experimental in the same way they are allowed to do so with a repairman-inspection certificate in the E-LSA realm. But that's a tangent, it still doesn't address the acquisition price non-starter between the few 4-seaters in EXAB land and old certified 4 seaters. As much as I detest the certified rules, and have my seasonal bouts with wanting to chuck the thing, the reality is that I'm orders of magnitude of money ahead with the spam can on a total yearly outlay than attempting to capitalize an RV-10. Not even close. That's unfortunately the way the cookie crumbles in my world where cost is an object. It is what it is. It is certainly no small part of why the hobby is dying with the younger generation, and I digress.
1a) To be clear, the cost delta to assemble a 4 seater and a 2-seater fixed gear airplane is trivial. The engine choice cost is also trivial. Yes a 4-seater would call for a bigger engine to be competitive, but a Lycoming 540 is no more expensive to overhaul in its parallel valve variant than a Lyco 360 of angle valve variant. Hell, a 540 is cheaper to overhaul than a Lyco 390, with insanely priced jugs. BL, it's not a materials or engine cost. It's merely a CAPEX one because of the prevailing depreciated price of certified 4-seaters.
1b) Very few builders are interested in assembling the equivalent of an experimental grumman Tiger (essentially a 4-seater RV, aka putting a 4 banger on and RV-10 and making it a 3-seater like all sub-200hp certified 4-seaters are and why they're priced the way they are). Nobody in builder land does that, which is why there's no affordable 4-seaters in EAB. The only other options are oddball Velocities with T-38 runway requirements, equally horrid high DA performance specs and family unfriendly cabin volumetrics, or hen's teeth fiberglass constructed offerings in the orphan "plans-built" market I wouldn't strap in on if the choices were that or get COVID-19.
2) Demographics. Good bad or indifferent, the preponderance of the experimental market is empty nester boomers, or childless couples. The heads of household who do partake do so where the decision was made the spouse was not interested in flying, or there was never going to be a willingness to travel with the children. As such, the market essentially settled on 2-seaters, for those without the aptitude, inclination and/or time to build in order to escape certified hell. See 1b for the feedback loop of why demographics feed the outcome of the offerings. Everybody else with flying-friendly families is stuck in certified land, myself included. Conversely, the few heads of household with young children have no interest in building outright as a function of life stage financials and time, which makes it a catch-22.
That's a good initial crack as to why there's no affordable options for 4-seaters. The current trend in certified is the loss of airframe OEM support until it becomes uneconomical to maintain. I won't mention specific make/models as that just triggers the type cults. The fixed gear trainer-lineage airplanes like C1xx and non-retract PA-28s will endure for a long time because of the sheer number of salvage samples and active flight training market that still enjoys 3rd party vendor support. But the antiquey retracts and twins are in a world of hurt already, and maintaining them under certified rules going forward will continue to be an exercise in watching a terminal patient decay without having someone mercy kill it already. To each their own on that.
It's always been a lone-wolf hobby in practice, and current rules don't help the cause. I have my theory as to why the FAA doesn't want to release certified cans into primary non-commercial, but that's my conspiracy theory and for another thread.