I’ve been told that you can actually see St Pancras from Euston,
I suspect that is technically true, but highly misleading.
There does appear to be a straight line with no obstructions from the side wall of Euston to the Side wall of St pancras down Pheonix road/Brill place but it's not opposite any of the entrances to Euston and given the distances involved and the fact that the road is built up on both sides. you will be seeing a tiny patch of non-descript wall that happens to be part of St pancras.
I suspect if you stood on the roof of Euston you would
I can only assume that passengers are incapable of walking in a straight line from Euston to St Pancras because Euston is such a hateful brutalist concrete environment that all passengers leaving it are too disorientated.
In a City, unless you have a wide straight road, you can't see very far. The buildings just get in the way too much, this applies regardless of whether they are "hateful and brutalist" or whether they are buitiful architecture, it's just the nature of being in a city. British cities also tend to have roads that are not at right angles making it easy to loose track of which direction you are facing.
There are a multitude of roads heading east in the vicinity of Euston station, but only two of them actually provide a workable route through to St pancras.
* Euston Road goes through, but it's some way south of the actual rail stations. Still it's probably the "easiest" route.
* Grafton Place (roughly lined up with the bus station), goes one block to a T junction with a row of buildings and locked gates in front of you.
* Wellesly Place is a tiny stub,
* Lancing Street (opposite the steps down from the station frontage) goes about half a block before hitting locked gates and private signs.
* Dorrik way and Drummond Crescent form a loop. There is an alley called churchway from the end of the loop which takes you a bit further east. Continuing east from there you reach an unnamed alley with gates that does not appear intended for public use. Even if you could get through there you hit a dead end.
* Pheonix road does go through for pedestrains (but not cars), and forms part of the signposted walking route, if only they did a better job of signposting it.
* Polygon road takes you east as far as Brill park, but then you hit a wall.