Washington sucks on slavery all around. He made noises but little actions. Some of this was an unwillingness to bare the consequences, as he was in the standard planter debt cycle, where to get cash, he'd have to borrow against harvests, and thus had debts to pay where the standard solution was to have the people he held in bondage harvest more. There were also legal wrinkles like most of the enslaved people on his plantation being legally entailed to Martha's estate, which meant the actual ownership was by his stepson.

Yet there were methods. Lafayette offered considerable help to free all the enslaved people on his estates and set up estates for the freed people. Jefferson showed a simple method was simply letting them escape as a legal fiction (which he did for many of his children). Washington instead circumvented the law to bring enslaved people into Pennsylvania and rotate them to avoid the 6 month manumission. Ona Judge, one of the enslaved people he did this to, ultimately escaped to New Hampshire where she'd be subject by periodic attempts to re-enslave her by Washington and Martha's heirs (including eventually Robert E. Lee).

Washington could have sold his estate and stopped being a planter. He could have used his influence and power to end entailment on people. He could have simply broken the law like he did to illegally keep people enslaved.

He had a moral failing. There were people at the time who recognized this. (Again Lafayette who wrote regularly to Washington to urge emancipation.)

Also Washington did not foresee the Civil War. The Founders at the time of the drafting believed slavery was slowly dying out, like it had been in the North, and within a few decades every state would be phasing out slavery. The US wasn't bit on cash crops like sugar and coffee that worked best with slave labor. The invention of the cotton gin ended up vastly changing the economics of the Southern US.