I know the democrats think "cracking down on businesses that hire illegals" is the solution to the problem, but that's an insanely difficult task, which is why it's more evidence that they don't want a solution. They're illegal immigrants, so the ones getting paid under the table are doing it in secret, making it difficult to investigate with little or no paper trail, many use false identities for employment, so that doesn't really fall on the employer either. Even if they were somehow miraculously able to keep people from hiring illegals, that would just leave a ton of illegals here with no income other than crime.
Mexico doesn't have to write a check in order to pay for it. They will lose that money in remittances which will be spent in our own economy instead, which more than covers the cost of the wall. The wall is also a one-time cost, rather than programs with a perpetual cost.