Why can’t women just continue serving without being ordained, like they do now?

In order to answer this question, it is helpful to understand what an ordination means in an Orthodox context. In one sense, we are all “ordained” into the ministry of Christ—the Royal Priesthood—by virtue of our baptism and chrismation. In a more specific sense, an ordination is a setting apart of people for ministry in a particular community, changing their relationship with the community. Having been recognized by the community, their gifts are then enlivened by the grace of the Holy Spirit. Those elevated to “priesthood” (i.e. bishop, priest, and deacon) are ordained in the context of the Eucharist, at the altar, and by the bishop. Their service is tied to the liturgy and the altar as the source and summit of their ministry. In concrete terms, they connect the liturgy of our lives to the sacramental life of the Church.
With the blessing of the bishop, their ministry is universal in scope and confers the authority, credibility, recognition, support, and protection of the Church. It also demands public accountability to the Church and obedience to the bishop. Lay ministry is important. However, it does not function in a sacramental capacity in the way that an ordained ministry does. (For instance, a lay chaplain can visit and pray with the sick, but cannot bring communion to them.) In addition, the authority, recognition, support, and protection of the Church given to the lay minister can vary greatly. Furthermore, the accountability of the lay minister to the Church can vary as well. (This can even be dangerous to those who are being served in areas where accountability and proper training are particularly important (e.g. pastoral care and spiritual direction.)) Moreover, lay ministry is typically more local in scope and usually dependent on the local priest for its exercise. If the local situation were to change (e.g. a parish gets a new priest who may have a different idea of what a lay minister, in particular a woman, can or should be doing in the church), the ministerial possibilities for her can change abruptly without recourse.