Hi Undomeher 
You’ve already go a lot of great answers here but I wanted to chime in with my thoughts and answers to your questions, too 
Why do we need so much protection? – I’ll echo what Pedros has already said (because he beat me to it), that when we open ourselves up to the energetic world and start working with said energy, our energy and workings can act as a beacon to other spirits around us like a lighthouse. It’s not necessarily a magnet, per se, but it is a light that shines in the darkness that brings attention to the rocky cliff ahead. Those spirits can then either go toward the lighthouse, toward our energy, or steer clear of it. This in turn can lead us to have more spiritual encounters, more bad luck or malevolent encounters, and even more beneficial encounters.
There’s also unintended curses or hexes that we can be given such as the Evil Eye 
that we can protect ourselves from. We never really know the intentions of others so I think protection is important. I also like to look at it this way → I am heavy into many different spiritual and witchcraft communities. One thing that some people like to do is hand out blessings left and right. I see it all the time, May Hera bless you! or May Hermes guide your way, and while these blessings are usually intended well, not everyone wants or needs the blessings of those spirits. Having protections in place can keep those particular blessings away, i.e. someone can have their deity of pregnancy bless me but my protections are gonna shut that down real quick 
Are more witches more susceptible to negative energy? – Again, I don’t think witches are more susceptible to “negative” energy (and energy isn’t inherently negative, either). We just happen to be more aware of it around us. It’s that awareness that gives us pause to protect. While we do all these protective spells and wards, no one really bats an eye at the nightly prayers of Christians, prayers before sporting events for the protection of the players, etc. That’s more visible, in my opinion, but I feel like what most witches do is on the same level.
And if we’re about love and light and harm to none and the threefold rule, how is all this negative energy accosting us? – Not everyone is love and light or follows the Wiccan Rede, as @BryWisteria pointed out. And someone can be as love and light as possible, but we’re all still human. We can still get envious and jealous, throw the evil eye unintentionally, mutter a curse under our breath – those things all hold weight and power whether we acknowledge them or not.
I also agree with @Valen too because we see protection spells everywhere. Everyone is constantly posting new protection spells, jars, trinkets, and amulets but sometimes we can go overboard. I have some wards up around my home, a protective piece of jewelry that I wear, and a charm I carry in my car. Other than that, I don’t do much else unless I know I’m going to need it. My wards and stuff do tend to be more on the heavy side because I’m in the public eye with YouTube and my podcast, so I do take extra precautions, but those aren’t going to be necessary for everyone.
Also, I saw that you had an accident and you hit a deer! I’m glad you’re okay, but I wanted to plug my travel protection charm maybe you can create something similar and carry it with you in your car in the future.
