It's not that easy. Poles vs trenches are a tradeoff discussion. FWIW I was once in construction digging trenches and I'm German, so I might be biased a bit.
Pro poles / open air:
- very, VERY cheap and fast to build out with GPON. That's how you got 1/1 GBit fiber in some piss poor village in the rural ditches of Romania.
- easy to get access when you need to do maintenance
Con poles / open air:
- it looks fucking ugly. Many a nice photo from Romania got some sort of half assed fiber cable on it.
- it's easy for drunk drivers, vandals (for the Americans: idiots shooting birds that rest on aboveground lines [1][2]), sabotage agents or moronic cable thieves to access and damage infrastructure
Pro trench digging:
- it's incredibly resilient. To take out electricity and power, you need a natural disaster at the scale of the infamous Ahrtal floods that ripped through bridges carrying cables and outright submerged and thus ruined district distribution networking rooms, but even the heaviest hailstorm doesn't give a fuck about cable that's buried. Drunk drivers are no concern, and so are cable thieves or terrorists.
- it looks way better, especially when local governments go and re-surface the roads afterwards
Cons trench digging:
- it's expensive, machinery and qualified staff are rare
- you usually need lots more bureaucracy with permits, traffic planning or what not else that's needed to dig a trench
- when something does happen below ground, it can be ... challenging to access the fault.
- in urban or even moderately settled areas, space below ground can be absurdly congested with existing infrastructure that necessitates a lot of manual excavation instead of machinery. Gas, water, sewers, long decommissioned pipe postal service lines, subways, low voltage power, high voltage power, other fiber providers, cable TV...
[1] https://www.usgs.gov/news/national-news-release/illegal-shoo...
[2] https://ucs.net/node/513