My wife and I make the same, all w-2 income & file M1 on our W4. Why do we owe a five-figure amount in federal tax beyond what was withheld by our respective employers?

For two earner families, and especially as your income gets higher, the withholding tables do not work very well. You can file new W-4s with your employers and choose "married but withhold at the higher single rate" and claim 0 exemptions. Even that may not have enough withheld, so you can also ask your employer to withhold an extra cash amount each pay period. I fought that for years and both my wife and I had to withhold at the single rate, claim 0 and have extra cash withheld each pay period.