Perhaps you have been playing long enough to remember, maybe you are a bit newer, or maybe you don't read patch notes. All the same, it doesn't change the fact that bot tower has no fortification buff in the earliest levels of the game.
To understand what this means, we need to define what fortification is (this is for you newer players), and why the change was made in the first place.
Fortification is a buff that makes it so your attacks only deal 50% damage reduction from all sources, the reduction applies before armor; this effect expires on the top and mid tower at 5 minutes.
On patch 6.15 there was a couple of changes to fortification; the important ones are as follows:
-> Duration reduced to 5 minutes from 7.
-> Damage reduction increased to 50% from 35%.
-> Bot lane turrets no longer have FORTIFICATION
Let us take a moment to understand what on earth was going on in S6 to understand why Riot would have done this.
For those not playing the game at the time, Season 6 was the first season that introduced Lethality, and the marksmen who abused it. At the time, it was win bot win game. Sounds like S7 and Ardent Sensor right? Well not quite; as an ADC main, I would call this the beginning of the decline of the golden age. The time when my team *actually needed me to take towers*. You see, at the time mages couldnt deal *AP* damage to towers, like they can now. The Vladimir with 600 AP can probably take the tower in a few autos now, chunking it each time, vs the marksmen who will look like they are struggling.
The purpose to taking Fortification off of the bot towers was an effort to get Junglers to spend more time pressuring the lane and getting them to rotate out faster to other lanes. Then in S7, this stayed true as you then saw the S7 Ardent meta the season after.
Once mages had tower pressure, not having resistance on the bot tower felt kind of pointless. Honestly, I was personally surprised to find that fortification wasn't removed when turret platting was added.
Not that I think its something so pressing it needs to be removed, but just as a reminder, its still there. :)