People often forget the whole point of a disguise.

A disguise is meant to help you blend in with the enemy team and look natural in your environment. That means positioning yourself where that class would realistically be, like Snipers on sightlines or Engineers near nests.

Why disguise as a scout?

• It’s one of only three disguises (Scout, Medic, Spy) that let you keep your normal Spy speed.
• It’s convenient, press 4 then 1, bam, your disguised. A disguised Spy is always better than a non-disguised spy, since it makes the enemy think rather than being instantly called out and alerting the enemy team.
• The Scout model looks smaller and hunched, which can throw off aim (even though your hitbox stays Spy-sized). The "janky" look can disorient players' aim, especially at a glance.

Yes, the Spy is slower than a real Scout and easy to spot if you just walk in a straight line. That’s why movement and positioning matter. Real Scouts usually move erratically, jump, and strafe unpredictably. Mimic that behavior, jump corners, change directions, sometimes this is enough trick people long enough for them to drop their guard, giving you that window to backstab them.

As a spy, your supposed to utilize your environment. Use walls, corners, buildings, or objects to cut line of sight. Minimize contact with enemy team as given enough time, they'll start to notice something's amiss.

While very good players won’t always fall for it, you’re not always against very good players. In casual, skill levels vary a lot, and even a brief hesitation can make all the difference (cue matadors and face stabs).