Most anti-war politicians have not, because the withdrawal from Syria does not really represent a real withdrawal or a significant decrease in American military presence in the Middle East.
As the AP reports (thanks to @Fizz for pointing this out), Defense Secretary Mark Esper has announced that the troops being withdrawn from Syria won't be coming home to the US: instead, they're going to Iraq to continue fighting ISIS:
[Defense Secretary Mark] Esper’s earlier comments to reporters traveling with him were the first to specifically lay out where American troops will go as they shift from Syria and what the counter-IS fight could look like. Esper said he has spoken to his Iraqi counterpart about the plan to shift about 1,000 troops from Syria into western Iraq.
Esper said the troops going into Iraq will have two missions: help defend Iraq and perform a counter-IS mission.
Esper discusses keeping small US force in northeast Syria - The Associated Press
At the same time Trump announced that he was withdrawing 50 troops from Syria, he announced the deployment of 3000 additional troops to Saudi Arabia for potential use against Iran:
WASHINGTON — The United States is sending about 3,000 additional troops to Saudi Arabia in the latest military response by the Trump administration after it accused Iran of attacks last month on Saudi oil facilities, the Pentagon said on Friday.
The move came only five days after President Trump said that his desire to terminate America’s “endless wars” led to his decision to pull back from the border area between Syria and Turkey about 50 troops who were working to create a “safe zone” between Turkish and Kurdish troops.
Trump Orders Troops and Weapons to Saudi Arabia in Message of Deterrence to Iran – The New York Times
As of the end of October, it seems people's skepticism was well founded: as The New York Times reports, Trump has reversed course and is increasing the number of US troops in Syria, back up to a number near the original level:
WASHINGTON — Every day in northeastern Syria, waves of American troops are pulling out under President Trump’s order this month that paved the way for a Turkish offensive that included assaults on the Pentagon’s allies, the Syrian Kurds.
And at the same time, a separate wave of American troops from the opposite direction is pouring back in.
In fact, once the comings and goings are done, the total number of United States forces in Syria is expected to be about 900 — close to the 1,000 troops on the ground when Mr. Trump ordered the withdrawal of American forces from the country.
...
By Oct. 20, things were shifting again. Mr. Trump was talking about the need to protect the oil fields in eastern Syria. Pentagon officials began working on a plan to send additional American troops to guard oil fields.
When combined with the troops at Al-Tanf, that brings the number of American troops projected to be in Syria to near 900, a number that could easily rise if, as expected, the Islamic State begins to make a comeback.
Hundreds of U.S. Troops Leaving, and Also Arriving in, Syria - The New York Times, 10/31/2019
So, in the end this so-called "withdrawal" appears to be little more than a troop relocation.
And finally, even if US troops had been withdrawn from Syria, the initial plan was only to withdraw 50. The US expanded the withdrawal plan only after pressure from Turkey and Turkish-backed militias:
Less than 48 hours before the withdrawal announcement, U.S. Gen. Mark A. Milley, chairman of the Joint Chiefs of Staff, had given assurances that the troops would remain indefinitely, standing by their Kurdish partners to continue to hunt down the Islamic State.
But the Turks’ capture Sunday of a key highway that served as the U.S. troops’ main supply line revealed the fragility of [their] mission
The hasty U.S. pullback from Syria is a searing moment in America’s withdrawal from the Middle East - The Washington Post
In summary:
- The troops aren't being withdrawn – they're just being moved to another war zone to continue the same mission
- Instead of decreasing, the number of US troops in the Middle East has actually increased, and they've done so in a way that represents an escalation in tensions with Iran, thus making war more likely.
- The so-called "Withdrawal" appears to have been little more than a troop relocation. As of November, the total number of US troops in Syria is little changed from the number that were there before the "withdrawal"
- Even if US troops had actually been withdrawn from Syria, many would question how much credit Trump should get, since that plan seemed to have been driven more by circumstances than any actual plan or desire to bring US troops home.
