My back bets keep losing, why would a bookie gub me?

Bookmakers are not interested in your short-term results. They care about the long-term.

When you are matched betting, the expected value of your bets is in your favour. That means even if you are currently losing at the bookmaker, you will eventually come out ahead over time because you are consistently placing +EV bets. Bookmakers understand this, which is why it is in their interest to restrict your account as early as possible, before that edge has time to show up in the numbers.

The roulette table is a useful contrast. In roulette the expected value is in the casino's favour. So even if a player goes on a hot streak and wins thousands, the casino is happy to keep letting them play, because they know the edge will eventually work in their favour over enough spins.

Matched betting flips that dynamic. You are the one with the long-term edge, which is exactly why bookmakers want to shut you down.