Icedove (Thunderbird) works well enough for me. Maybe the reason it’s “old fashioned” is because it works well enough that it doesn’t need to be changed that often.
In the proprietary software world we’re used to UI’s being redesigned on a regular basis for no user benefit.
Same reason anyone would use a dedicated provider-independent client instead of a proprietary web application locked into a single provider: less vendor lock-in, more local control, and so on.