As it happens, McClellan was quite in favour of them, but the problem was manpower.McClellan and Halleck after him (and even more so) were not fond of very deep envelopments, evidently. The enclaves along the coast were never really followed up with anything deeply penetrating, even though many of the local commanders could see the wasted opportunities. It blows my mind how often there was an opportunity to cut the rail line leading right to the ANV with no sustained efforts made to take it out (though, to be fair, rail lines would be repaired quickly, so it would have to be something sustained rather than a raid).
The problem was pretty much always manpower.
The calculus was always:
How many troops will we need to land along the coast to make a battle group capable of surviving whatever the Confederates might concentrate against us if they go inland? (Probably quite a lot.)
Do we have those troops to spare? (Almost certainly not.)
Would those troops be more valuable with the main army? (Everyone thought so yes).
If the perception is that the enemy army has slightly more men than you around Richmond, or that another 15,000 men will give you enough of an edge to be decisive, or even that if you reduce the strength of your army a bit it might be defeated... well, there's not enough men with the main army yet.
It's once the main army is big enough that the manpower can be spared for peripheral operations in support of it.
To give some sense of the scale of the problem, one analysis I saw suggested that the total strength along the Confederate coastline in early 1862 was:
2500 in Texas
7000 in Louisiana (facing 2000 Federals)
12000 in Alabama and Florida
22000 in Georgia and South Carolina (facing 16000 Federals)
12000 in NC
And 31000 around Norfolk and Yorktown (facing 12000 Federals)
That's just the regular Confederate troops - militia is separate, and has to be considered as well. IIRC Livermore gave some estimates for early-1862 Confederate militia strength, and there's several tens of thousands of potential militia troops across the Southern states. (Which is why the CSA was able to sustain so many deaths in the war without their armies hollowing out far sooner than they actually did.)
The Union Army in early 1862 has a very big field force in Maryland and northern Virginia, but historically so much was required for Washington to be considered safe that it drained away offensive superiority even with the North Carolina expedition and the Dept. of the South stripped down. The first bolus of extra disposable troops would probably have gone into boosting the main eastern army, and then the second into peripheral ops, but it's almost impossible to find a point at which the Army of the Potomac is considered to be big enough.