It's not an offset if it would have been that way anyway - In other words, you owning/living in a rural area does nothing extra to offset carbon emissions, because that rural area would have offset it regardless.
The infrastructure-to-population cost ratio of urban areas is way, way higher than rural areas. A meter of footpath, road or sewer in a city is reused by (just spitballing numbers) 10x as many people compared to a suburb, and probably 100x vs a rural area. Infrastructure is just much cheaper in cities relative to the served population, because so much less land area needs to be covered.
Since people generally pay the same taxes no matter where they live, the net result is cities generally fund a lot of infrastructure for other areas, as they are so much more efficient.
There's nothing necessarily wrong with this for genuinely rural areas - we do need farms etc, so infrastructure built to serve them is well spent. The issue arises when we sprawl with suburbs, as they don't serve any particular purpose that couldn't be fulfilled by a dense city, but cost much more in terms of infrastructure expense.
I have no problem with people living in suburbs, so long as they bear most of the cost of living that lifestyle, rather than dense urban areas funding them.