Why not just accept the deal, and then negotiate better terms down the road? Is there something binding in it that would make it hard to renegotiate once the pressure of the clock has been removed, and you possibly have better leverage?
There are many and various extremely good reasons for the backstop, but its very existence is anathema for many Brexiters, since it would force the UK to follow EU regulations indefinitely. Alternatively, the UK could put a customs border in the sea between NI and the rest, but that would put NI in a different regulatory regime, and that's anathema to the Tories' partner in government, the Northern Irish DUP, which will not accept anything that could move NI closer to the RoI. So 1) most Brexiters don't like a customs union and don't care about NI 2) the DUP don't want any borders on either "side" of NI (but want Brexit anyway).
https://www.apnews.com/eb12b6eb1da3436cba22bd64de6d9ec8
The actual contours of the UK/EU relationship after Brexit would otherwise be mostly up for negotiation. If you didn't mind a possible permanent customs union, then exiting with May's deal followed by a bunch more negotiations would seem like a sane thing to do...
There's also the Good Friday Agreement, which brought peace to Northern Ireland. A hard border would easily be seen as a betrayal of it and could plausibly cause it to collapse.
The whole thing is just an impossible trilemma. Can't have a border on the island, because RoI will not let the EU sign that deal. Can't have one in the ocean, because the Tories need the DUP votes in Parliament (they don't have a majority, and the government will fall if the DUP pulls out). Can't have a backstop because it could mean a permanent half-in half-out twilight where the UK stays in the customs union forever and hard Brexit parliamentarians can't swallow that. Three totally incompatible red lines.
https://en.wikipedia.org/wiki/Irish_border_question
The default, if no deal is signed, is a hard border on the island. Which is bad, but not obviously that much worse for the RoI than a deal with a hard border. So the RoI won't settle for a deal without a backstop.
https://www.irishtimes.com/news/politics/dup-doubles-down-on...
They certainly don't seem to want a hard border with RoI, but they will sooner chop an arm off than accept a hard border between NI and the rest of the UK, so given the choice I think they'd take the former. They're the Democratic Unionist Party, keeping NI an integrated part of the UK is literally their entire reason for existing.