This question is not your question, but I think it's relevant here for context: the answer by /u/PM_ME_SILLY_SECRETS points out that road surfaces in Roman cities suffered extensive wear and did have to be repaired to keep them maximally useful.

This answer by /u/IsOughtDistinction provides more information about the difference in purpose between modern roads: outside of cities, although our modern traffic has rubber tyres, Roman roads certainly saw much less traffic, much slower traffic and much lighter traffic at the axle than do our modern roads, and even then those roads needed maintenance or else they degraded.

While we wait for an answer as to the history of the situation I think it's worth thinking about this and a factor that relates to the survivorship bias you mention: ancient infrastructure was not necessarily built strong enough to last millenia on purpose. Modern skills allow a bridge to be designed to last 50 years and to be pretty sure that's how long it will last. If a Greek civil engineer designed a bridge that by their best estimate would last 50 years, it might last 50 - but it might fall down after 1 year or last 500 years, because engineering understanding and techniques were less certain. This presumably led to a certain amount of "overbuilding" to ensure success - raising the average lifetime of the structure in order to keep the "pessimistic probable lifetime" above the target - such as there was a target lifetime. (Hopefully that will be part of any eventual answer)

Speaking as an engineer, there's two relevant points that can be made to address this question.

One is that since the advent of mechanical calculators (and later, computer-aided design) and most importantly, precise theories on the physics of mechanics, both static and dynamical, but also important are thermodynamics, material science etc., and derived engineering formulas means we can calculate forces in a structure, and the necessary and load bearing to an incredible degree. This means that while we still leave some extra room in our equations to account for the inherent uncertainity and the human factor in construction plus the used materials, we can very accurately say said building will use this much material and will require this much work and this much time to build, thus costing this and this much overall, which is obviously the most important number for developers, ie people who actually pay for it. In turn we can also calculate expected usable lifespans, again an important metric for developers when considering their return to investment. So the lifespan of a building today reflects economic calculations, plus whatever regulations are in place.

Infrastructure, being public projects are somewhat different from a financing and requirements pov but still follow same general rules. That said most of our modern infr. doesn't really gets replaced, rather renovated. When you think of bridges to be replaced, you likely mean post-war bridges built in Europe where time constraints were tight and the expectation was that 50 years now Europe will be even so much richer so building replacements won't be a problem.

Our tools were not avalible in the past. While architects had access to some basic mathematical tools, made scale models and relied on geometry developped by Greeks, lot of it was good ol' trial and error and experience. Roman's didn't know how much an arch can really withstand exactly, but had a good idea from experience and usually designed and constructed extra buffer just to make it sure it'll stand - and in cases they haven't, well those don't stand anymore. De architectura by Marcus Vitruvius Pollio is our primary source on Roman engineering and worth looking into it, especially since it deals a lot with the ethos of architecture, and is far from being just a reference book.

Then what the other posters touched upon, the entirely different load our contemporary infrastructure has to withstand vs what ancient ones had to, which others already looked into so I won't here.