There are multiple supporting reasons Jehovah's Witnesses believe the Spirit is a force, rather than a person. Some are circumstantial in nature; others are more direct.
Definition This is a natural meaning of "spirit". Is the spirit of happiness a thinking entity? No. It's "the activating or essential principle influencing a person" (Merriam-Webster). Likewise, the spirit of God is the force, or activating principle, of God.
Source references The Greek New Testament only uses the neuter gender when referring to the Holy Spirit: it, not he. Furthermore, there the Spirit is never called by a "personal" name. Both are unlike the personal, masculine references to Jehovah (God the Father) and Jesus (Son of God). Why the differences? Because the Spirit is a thing, not a person.
Man-made Trinity JWs believe Trinitarianism is the product of the human Council of Alexandria and the Council of Constantinople. This isn't a direct reason to believe the Spirit is abstract per se, but it is a reason to distrust the common interpretation of God, which for mainstream Christianity differs little from these flawed councils.
Nature The Spirit is often referred to as a substance.
I shall have to take away some of the spirit that is upon you and place it upon them.
After that I will pour out my spirit on every sort of flesh, And your sons and your daughters will prophesy, Your old men will dream dreams, And your young men will see visions.
When have you ever heard anyone being divided in parts amongst people or poured on them? It's more akin to a substance than a person.
Consistent with human "spirits" Jehovah's Witnesses do not believe humans have a literal spirit or soul independent of a physical body. Our spirit is our life(force), not a ghost. If you believe that human spirits are abstract, it's easier to believe that God's spirit is likewise abstract. See "Soul" and "Spirit" for reasons for this belief about human spirits.
In relation to the scriptures you mentioned, JWs will point out
There are dozens upon dozens of such personifications in the Bible, including the three you mentioned. Using them to imbue every abstract concept like these with life would be inappropriate.