The southern border with Guatemala is ~ 540 miles long and and largely unenforceable. And the Mexican provinces there 'have a history' with their gov to put it mildly. Over 340 illegal cross routes there. You can rely on Mexico Gov less than you can rely on a $5 Mexican whore.
Factor in this: mass migrations has been a fave tool of Kremlin for ages. Bomb Alleppo and give them proper info, spread the rumours about the latest change in US Border patrol move, etc. and you have masses of folks naively buying into that with criminals among and around them. And US Congress is the most criminal of them all.
Trump is playing 2020 Games right so far...