1. The rates are actually better than many competitors, especially for the pickup 5*.
2. It has stellar writing by mobage standards, at least since Nasu took over. The events also always focus on groups of servants so everyone gets time in the spotlight.
3. 4* and 3* units are actually viable and people even make a show of clearing the final boss and challenge quests with 2* and 1* units. The game is one of the best in the market about preventing power creep by buffing old units and having non-5*s be viable.
4. The game also encourages you to use many different units with the class system and cost systems together, which also stops you from just making a party of 5*s with 5* CEs that you can use for all content.
4a. The events are actually interesting because they give buffs or drop boosts to certain characters which encourages you to use characters you wouldn't normally, and makes weaker units stronger than top tier units for the duration of them.
5. After the shitty first year, the animations significantly improved and look great these days.
I don't understand why people are constantly confused. FGO didn't become popular just because of the brand strength, it became popular because as far as mobage go it does almost everything right or at least better than its competitors.