Perhaps when the Met was built (1863) Euston was mostly used by long distance travellers of the better-off classes who would have turned up by cab. The suburban services and commuting traffic were not as important then. So the planners did not see much demand for interchange between the Met and the LNWR. Just a theory, although they made a better connection at King's Cross/St Pancras, and of course Paddington but maybe becaise the GWR was a shareholder.
Absolutely true - no well-off passenger was going to struggle to get into a Metropolitan Railway train at Gower Street having dragged his trunks from Euston. Incidentally, the shortest walking route would have been along Drummond Street to George Street and then south to the Met station.
I'm not sure connections were that good at Paddington. The original Metropolitan Railway station was at Bishops Road, (now the Hammersmith and City platforms) quite a long way from the Arrivals and Departures platforms in the main station. The Praed Street station opened a couple of years later in 1868.
In any event, in 1863, the northern limits of the built-up areas of London were just north of Regents Park, the bulk of the housing was south of the 'New Road' (now Euston Road).
Didn't the Met basically invent London commuting? You're likely right about Euston - most of the commuter towns towards and including Northampton grew massively in the 60s and 70s. Bletchley for example was a tiny rural market town in World War 2 when all the Bletchley Park stuff was going on.
ISTR even in the 1980s the weekday Northampton Line service was just 2tph.
The Met certainly invented commuting to the northwest of London, but very intensive suburban services were operated by the Great Eastern out of Liverpool Street at the end of the 19th and beginning of the 20th century which is described here in outline
https://en.wikipedia.org/wiki/Great_Eastern_Railway#Suburban_services_(the_Jazz). The railway companies operating south of London also offered intensive suburban service in the same period. None of these companies had the luxury of the longer distance routes of the main lines to the north and west with their more remunerative longer distance passengers so they had to build short distance custom.
None of the longer distance routes, Great Western, London and North Western, the Midland or the Great Northern were that interested in what would today be called 'commuter' traffic.