I've worked in the insurance industry for 15+ years.
The reason that you can't insure your house for $100k when it's actually worth $300k, even if you would rebuild a much smaller house, is because most losses are partial losses, and the predicted size of those partial losses is correlated with the full value of the house. A larger house can be expected to have more frequent and more expensive losses than a smaller one, all else equal.
First, I would caution against using homeguide. It's often behind the times and, in this inflationary environment, that's important. A $300/ft2 cost might be pretty accurate depending on your city/region.
Second, Traveler's lets me set the replacement cost, for whatever reason. It might help that my mortgage is very low compared to the value.
I have Traveler's and they wouldn't let me set it my own replacement cost. I don't have a mortgage.
Well, that makes no sense. If only a 2000 ft2 house gets built with the payout, wtf do they care? There's not a lien holder to complain.
I just talked to them and asked them exactly that and was told that it's illegal for them to insure someone's house for less than what it will cost to rebuild it.
Bacchi's initial point about the inflationary environment is correct; insurance companies are seeing rapidly rising costs to repair and replace insured homes, and supply chain issues with materials are also contributing.
The second point about whether you're allowed to pick your own replacement cost may have to do with state regulations. Insurance has virtually no federal regulation, it's all on a state-by-state basis, so while one state might allow a homeowner to pick their valuation, another might say that it's not in the public interest and prohibit it.
It's definitely worth shopping around if you've been with the same carrier for awhile. For those who have Travelers and an independent agent, that's what your agent gets paid their commission for. For anyone who has a carrier like State Farm, Farmers, Allstate, etc, your agent works for the insurance company and so really can't help you shop around, so I'd really recommend contacting an independent agent.