I can’t explain in complete detail now, but in the sentence you used above, you are expressing what you think is the will of another person. Typically when talking about other people’s thought, volition, or other things not easily observable in Japanese, the 〜ている form is necessary. 〜ようとする implies the exact moment when one will try to do something before it’s a continuous aspect. Since only you can determine that moment for yourself, it doesn’t make a lot of sense to apply that to another person.
When I get home I can provide more links, otherwise hopefully someone who is more available can explain this in more detail.
EDIT: I tried to look for the links where this is explained in English, but inexplicably I’m not using the right search query keywords to get those sites to come up quickly. Anyway, one example referring to talking about another’s volition can be found here. Although I’m aware it’s a different grammar point being referred to, it is a good place to point out that specifically when talking about other’s thoughts, feelings, or volition in Japanese one cannot use the same forms as you would for yourself.
Here are the two point I’d like to bring your attention to:
意志形は単独で使うとひとりごとや勧誘の意味を表わします。話し手がある行為をする意志があることを聞き手に伝える場合には、意志形に「と思います」(普通形「と思う」)をつけて使うのがふつうです。意志を一定期間持ち続けている場合は、「と思っています」を使います。
English:
When used by itself, the volitional form expresses the meaning of talking to oneself or an invitation to that nature. If the speaker conveys something that has their own will to the listener it’s normal to add と思います (と思う in plain form) to the volitional form. If one’s feelings persists for a constant period of time, one uses と思っています.
「(よ)うと思います」は、一時的な意志を直接表しますが、「(よ)うと思っています」は意志の持続を客観化して表わします。ですから、「(よ)うと思います」は話し手自身の意志しか表わせませんが、「(よ)うと思っています」は、聞き手や第三者の意志にも使えます。
English:
(よ)うとおもいます directly expresses temporary volition, but (よ)うと思っています expresses and objectifies the continuation of volition. Therefore (よ)うとおもいます can only express the speaker’s own will, but (よ)うと思っていますcan also be used for the volition of the listener and/or a third party.
So in other words, it could be inferred with this that ~ている has the ability to objectify the continuation of (something) so that it turns into an observation rather rather than an assertion that the speaker actually knows the listener’s or 3rd party’s volition, etc. I think this can be seen when people quote a 3rd party, they sometimes use ~と言っていました or even talk generally about what 3rd party thinks by using ~と思っています.
So going back to example sentence, ~ようとしています is an observation of what the speaker thinks another person is going to do. If that make sense at all.