We (the UK) don't want to control the movement of goods between Ireland and the UK, so that's not a concern to us.
We do want to control the movement of people, but we also don't want to cause tension - and perhaps eventually fighting/war by scrapping the Good Friday Agreement - so we have to consider the pros and cons.
We've determined that the installation of hard physical infrastructure at the border is likely to provoke violence so we think it better not to do that, and instead attempt to police the movement of people across the border by softer means. We can hope to find most people illegally living or working on the UK side of the border by means of employment records, tax returns, housing records, bank accounts and similar. Of course that won't catch 100% of illegal immigrants, but neither does a hard border. Admittedly the soft border will be more porous to illegal migration than a hard border but we consider that a price worth paying to hopefully preserve peace.
If the EU insist in installing hard border infrastructure on their side, then they need to raise the money to build it and convince the Irish that it's necessary. If it does get built and, regrettably, result in conflict and violence, then unfortunately both sides will have to deal with that. This would be a terrible tragedy so I hope the EU won't be so stupid as to go down that road; I don't have much confidence in the EU behaving sensibly though so I pin my hopes more on the Irish people and government refusing to install such a dangerous folly - regardless of the EU's demands.